A Comprehensive Appraisal of the Factors Impacting Construction Project Delivery Method Selection: A Systematic Analysis

ABSTRACT Construction projects may be procured by various means: design-build (DB), design-bid-build (DBB), construction management at risk (CM-at risk), and integrated project delivery (IPD), among many others. Each approach must be considered in selecting the most appropriate procurement strategy for any specific project. Thus, in making that choice, a selection approach along with selection criteria need to be considered. This study seeks to reassert clarity by consolidating the available literature on factors impacting project delivery selection and presenting a definitive, comprehensive list of those factors. The approach relies on systematic analysis. An initial retrieved 224 papers across 53 journals was reduced to 45 papers relevant to construction projects. These each offered between four to 39 factors. An examination and rationalization of themes distilled to a definitive 22 criteria and 35 sub-criteria. Chief among these, in descending order of reported frequency, were: owner related issues, contractor issues, time, cost, quality, contract disputes, risk, market conditions, regulation, complexity, innovation, scope, site, community support, project size, project type, resource availability, natural disasters, political impact, sustainability, culture, and safety. The factors will vary project-to-project, and a discussion of the impacts of these factors on the project delivery method selection is offered.


Introduction
The Project Delivery Method (PDM) selection in construction is a key step impacting project success (Dorsey 1997;Naoum 1994;Rwelamila and Meyer 1999). PDM describes how project participants are organized to interact in transforming the owner's goals and objectives into a finished facility . It directly affects construction performance, including delivery speed, cost, and quality (Al Khalil 2002;Diao, Dong, and Cui 2018;Noorzai 2020). PDM can be viewed as both a contractual structure and compensation arrangement by which project owners acquire a completed facility fit to their needs (Mafakheri et al. 2007). There are several PDMs, and the most common approaches are design-bid-build (DBB), design-build (DB), construction management at risk (CM-at risk) -also known as construction management as general contractor (GC), engineeringprocurement-construction (EPC), and integrated project delivery (IPD) (Li, Qin, and Li 2015;Qiang et al. 2015).
The appropriateness of the PDM selected dramatically impacts the performance of project implementation (Noorzai 2020). Past studies show that choosing the most appropriate PDM helps mitigate contract conflicts and cuts project costs from 10% to 30% (Hashem M. Mehany, Bashettiyavar, Esmaeili, & Gad, 2018). Some studies have concluded that the DB and CM methods offer greater advantages in terms of time and cost compared with DBB Molenaar and Franz 2018). Researchers have also shown that these advantages do not apply to all projects but vary according to project type (Demetracopoulou, O'Brien, and Khwaja 2020). DB can result in higher initial costs and fewer competitive bids when used on inappropriate projects (D. Q. Tran and Molenaar 2014). However, it is not an easy task to identify the most suitable PDM due to the large number of uncertainties that can surround projects at the implementation stages .
In selecting a PDM, determining the appropriate selection criteria and evaluating the effectiveness are vital first steps. The selection criteria of the PDM remains an active theme in project management research due to ever-evolving considerations and increasing project complexity. As early as 1985, NEDO identified nine criteria for project delivery selection (NEDO 1985). Since then, researchers have enriched this knowledge field, adding numerous variations to the criteria list. However, while these various lists enrich understanding of the factors impacting PDM selection, their divergent and inconsistent emphases have in fact brought some confusion to the process of PDM selection itself. It is thus timely that the body of knowledge available on PDM selection is interrogated with a view to establishing a comprehensive list of criteria, and in so doing, rationalize the tools available in the task of PDM selection. This study attempts to comprehensively identify and analyze previous work in this area and identify a clear set of criteria relevant to PDM selection.

Major delivery methods in practice
There are various methods of project delivery used in the construction industry. Of these, the design-bidbuild (DBB) -also referred to as the "traditional" delivery method -includes three key stakeholders: owner, designer (architect), and general contractor (builder). In this configuration, the owner monitors and controls the project performance of the designer's and contractor's activities to assure adherence to contract requirements (Mahdi and Alreshaid 2005;Victor Sanvido and Konchar 1999).
In the design-build (DB) method, the owner contracts with a single entity responsible for both design and construction (Noorzai 2020). This is a preferred option when a single source of responsibility and accountability is desired by the owner. In particular, as it is a single entity that is responsible for both design and construction, the adversarial relationship that can arise in DBB is obviated. (Mafakheri et al. 2007).
In the construction management/general contractor (CM/GC) format, the construction manager is the general contractor hired by the owner to provide consultation regarding the project's operational and financial aspects. Nevertheless, there are still two separate design and construction contracts to be managed. Thus, the construction manager is responsible for providing consultation on architectural services in evaluating costs, schedule, materials, and the like, and advising on optimizations and design alternatives. A construction manager is also responsible for monitoring and controlling the construction process in terms of costs, time, and other requirements to ensure a guaranteed maximum price for the project (Mafakheri et al. 2007;Victor Sanvido and Konchar 1999).
In the CM/PM format, the owner outsources all or part of the project management process to a program manager (or program management agency). The program manager could be the project manager for the entire process or the owner's agent to support and supplement the facility management expertise. It should be mentioned that program management tasks could be delegated to any or both design and build entities (Mafakheri et al. 2007).
Construction projects are traditionally based on transformation theory (Koskela 1992), which can also explain PDM. The difference between different PDMs lies in the work breakdown and the allocation of responsibilities and risks in the implementation process. Participants at each stage focus on transforming resources into project deliverables to increase the value of the project (Koskela 2000;Tuholski 2008). However, the shortcomings of the transformation theory are also obvious. When each participant tries to transform best individually, it often ignores the next activity or end customer's need. In addition, transformation theory does not involve how to use resources most effectively. These defects have led to a large amount of information loss, rework and waste (Bølviken, Rooke, and Koskela 2014). This is also one of the reasons why many delivery methods are gradually developing towards integration.

The selection of project delivery method
The owner usually determines the type of PDM prior to the start of the project. Decision-making is a complex process that arises from the uncertainty of the decision-making environment and the construction project itself . In point of fact, the PDMs selection problem can be considered a multi-criteria decision making problem (Li, Qin, and Li 2015). Previous researches on the selection of project delivery systems consist broadly of two types; the selection approach and the selection criteria. On the one hand, the selection approach research focuses on developing a selection model, typically transforming fuzzy judgment into an intuitive and easy-to-operate procedural process Li et al. 2019;Zhao et al. 2017). The selection criteria research aims, on the other hand, to find, weigh and examine criteria considered in practice (Demetracopoulou, O'Brien, and Khwaja 2020;Mosley and Bubshait 2017).
There is no "one-size-fits-all" best project delivery method. The selection of the project delivery system should be based on specific project requirements, specific characteristics and circumstances of the owner, and the successful formulation of the project team (Moore 2000). In determining the PDM, it is essential to identify appropriate selection criteria and evaluate their effectiveness. The selection criteria of PDM have always been a hot topic in the field of project management, and with the complexity of project development, it has been evolving, subdividing, and concretizing. Criteria may include but are not limited to technical excellence, management capability, financial capability, personnel qualifications, prior experience, safety, scope alternates and optional features offered, project completion data, and risk to the owner organization. A list of selection criteria can improve identifying the project features and selecting a more appropriate delivery process.
Although some studies have compared various delivery methods and come up with meaningful recommendations, these studies generally use the method of case comparison. For example, Alleman et al. (2017) compared 291 U.S. highway projects that alternative methods performed better than traditional methods. (Molenaar and Franz 2018) continued the work of CII(V. Sanvido and Konchar 1998), compared 212 projects using DBB, CMR, and DB, and compared the cost and schedule performance of these delivery methods. It is believed that DB has the best schedule performance, and the completed unit cost of the DB project is also comparable to that of DBB and slightly lower than CMR projects.
These studies provide meaningful guidance for owners' decision-making. However, the owner cannot simply apply these conclusions and need to find a suitable delivery method based on some factors. Choosing suitable factors from a list of influencing factors as comprehensively as possible is the basis of the decision-making method and the premise of evaluating the performance of the project delivery method.

Research methodology
The selection of a PDM depends upon many factors. This research aims to identify indicators by a comprehensive literature review. A systematic approach is adopted to review and analyze the findings of previous studies related to selection criteria of project delivery systems.
A series of preparatory work was carried out before the formal work, including a preliminary reading of the literature, identifying similar keywords, and identifying research areas. There are different expressions of project delivery methods in various works of literature. Therefore, the preparation work before the study first identifies these expressions, and through the previous literature reading, identifies five similar presentations such as "project delivery method/system," "project procurement method/system," "project contract system." Additionally, because the words "project" and "delivery" have different meanings in different fields, the two words are combined to avoid the search scope expanding too much.
A systematic approach is adopted to review and analyze the findings of previous studies related to PDM selection factors. The methodology adopted follows a similar approach to that used by Ke et al. (2009); (Yuan and Shen 2011). It involves a three-stage approach, including six steps, as illustrated in Figure 1.
In the first stage, a comprehensive desktop search was conducted under "T/A/K (title/ abstract/keyword)." The main purpose at this stage was to ascertain the level of attention afforded PDM in the research literature body, as well as establishing boundaries of the study. Three mainstream databases, Science Direct, Scopus, and Web of Science, were finally determined through preliminary preparation and screening. These databases contain a wide range of documents in different fields, reducing omissions. Keywords such as "project delivery system," "project procurement system," "project contract system," "project procurement method" were entered in the "T/A/K" section in each database, respectively. The article is limited to "article"  and "review" and published before March 2020. The whole search code and results in each database are listed as followed in Table 1.
In the second stage, firstly, the same articles searched from different databases are merged. The searched articles involve many fields, including construction management, energy, computer science, health care science, etc., then visual filtering of articles was conducted from a Construction Management perspective. The search retrieved 224 papers across 53 journals.
Titles, abstracts, and keywords from the retrieved papers were examined to determine content pertinent to factors influencing the selecting PDMs in the construction area. Documents that are not related to the selection criteria of the project delivery method were filtered out, such as the PDM efficiency, contractor selection, risk allocation, etc. At the same time, a research report by NEDO and a conference paper by PMI were included in the list because of the earliest proposed indicators and their impact in practice, respectively. Overall, 45 papers were identified for further analysis. The articles, journals, and authors are listed in the appendix.
In the third stage, the remaining 45 papers were subjected to content analysis to identify factors impacting project delivery choice and scrutinized. Figure 2 present the selection criteria identified from the selected publications. The PDM selection criteria are effective strategies that augment project success. The authors identified 784 criteria across 45 papers, with individual documents offering between four to 39 factors. Following systematic analysis, these were rationalized into 22 criteria and 35 sub-criteria. In the analysis process, items with the same or similar meanings were folded into a single term class . For example, regarding the criteria "time", different studies used expressions such as speed, schedule, and milestone. These were grouped under the same broad category while holding onto any distinctions within the sub-criteria. See Table 2. Table 3 shows the journal sources of the 45 selected papers. Although there was no restriction on the journal's scope when searching for the target literature, the results of the visual review show that the top six journals were in fact the top international journals in construction engineering management (Chau 1997). The top six journals total delivered 26 articles, accounting for 58% of the total retrieved studies. See Table 3. Figure 3 shows the trend of the criteria proposed by the researcher over time. Since NEDO proposed nine PDM selection criteria in 1985, scholars have continuously enriched this indicator system, increasing the total number trend. The rigid requirements of time, cost, risk, quality, safety, and owner responsibility, originally closely linked to the project management system, have gradually expanded to community participation, political and government regulation, and the external environment. This is in line with the trend of increasing scale and complexity of construction projects. Large-scale and complex projects require project organizations and members to cope with more challenging environments. As in cross-regional (including cross-border) projects, the organization and collaboration of internal project members are more restricted by external laws, local support, and cultural  factors. Owners and contractors must abandon some requirements to balance these constraints when determining delivery methods. For example, although the DB method can achieve a shorter construction period, the method has only a few entities with management rights. The local government may require an increase in the participation and share of local enterprises in the project management organization due to the expectation of improving the competitiveness of local enterprises. Therefore, the general contractors need to make some compromises, abandon DB and switch to other delivery methods. Figures 4 to 6 reflect the evolution of research papers on selection factors based on project types. The research on general project selection factors still dominates in terms of the total amount, with 27 articles accounting for 60%. However, the increased researchers realize a strong relationship between the type of project and the choice of project delivery method. Each project in every industry has characteristics that may influence the choice of project delivery method (Touran et al. 2009). Research on the selection factors of specific types of projects is receiving widespread attention. This dividing line appeared in   Figure 5 shows the number of selected indicators in each article considering the project type. The number of selection criteria for general project delivery methods reached 34 criteria, while the selection criteria for transportation and highway engineering reached a maximum of 39 criteria. Figure 6 reveals the number of articles based on project research types around 2010. Before 2010, the research on the selection factors of general project delivery methods dominated, reaching 15 articles, accounting for 88%. Only two articles looked at building and water facilities. After 2010, there were 28 research articles, and the specific project types gradually increased to 57% (16 papers), while the proportion of general projects dropped to 43% (12 papers). A total of seven types of projects have been studied, of which transportation engineering, including highways, has become the dominant research object, a total of 10 articles accounting for 36%.

Owner related issues
Owner-related issues were ranked first in all factors and were mentioned 127 times. Owner's issues include owner's role and responsibility (NEDO 1985;Ng et al. 2002;Qiang et al. 2015), owner's ability (Gordon 1994;Luu, Ng, and Chen 2003;Xia et al. 2013), owner's willingness to participate Demetracopoulou, O'Brien, and Khwaja 2020;Luu, Ng, and Chen 2003), owner's type Luu, Ng, and Chen 2003), owner's risk preference Luu, Ng, and Chen 2005) and mutual trust (Luu, Ng, and Chen 2005;Qiang et al. 2015). DBB is more adopted when owners want to be dominant in management and exercise more rights. The government or other public sector owners may intend to choose DB or CM method because they are unable to participate in project implementation in- depth due to lack of project construction experience and control capacity. If the owner and contractor cannot trust each other, the owner has to spend a lot of money and energy to monitor the execution of the project. The owner's sufficient technical knowledge and experience is also a factor that owners need to assess. Although it is not necessary to have the same knowledge and capabilities as the contractor, the owner must still have sufficient knowledge to understand the contractor's behavior and communicate well. All these characteristics of the owners inevitably affect the relationship with the contractor, the allocation of risks, and the project management contract. The owner shall define the scope and extent of his authorization and establish the basic framework of the contract.

Contractor related issues
Overall, contractor-related issues ranked second among all factors and were mentioned a total of 114 times. Some works of literature list competitive contractors in the top ten most influential parameters (Luu, Ng, and    Qiang et al. 2015). The availability of contractors needs to consider the number of potential contractors on the market and the degree of market information accessibility. Contractor's capabilities and experience include but are not limited to similar project experience (X. Ding et al. 2014), experience with a particular delivery method (Lopez, Mascione, and Liu 2017), the ability to use advanced technology (Marzouk and Elmesteckawi 2015), experienced employee acquisition (Dai Q Tran et al. 2013), labor productivity (Luu, Ng, and Chen 2003), ensuring the availability and stability of the supply chain (Moon et al. 2011), construction arrangement, such as adjustment and integration of activity sequence, and supervision, etc. (Alleman et al. 2017;Demetracopoulou, O'Brien, and Khwaja 2020;Lopez, Mascione, and Liu 2017;Noorzai 2020). In addition, the sooner the contractor participates in the project, the more efficiently the project can be implemented. As the survey conducted by Touran et al. (2011) found, the early involvement of construction contractors through CMR contracts not only establishes a budget earlier than DBB but also provides designers with realtime cost estimates and constructability analysis to improve the project's bottom line.

Time and scheduling
Time is still a significant constraint and objective in most projects and is mentioned 63 times in different expressions. The criteria can be expressed by time, schedule or speed, etc., and be divided into time certainty, time control in phases, and the time requirement. When signing the contract, the owner and the contractor need to agree to the degree of certainty that they will complete the project on the date (Ng et al. 2002). Different PDMs have different abilities to shorten the time caused by the organization relationship and operation mode, and the project owner needs to judge which delivery method has the most outstanding ability. On-time completion (Luu, Ng, and Chen 2003)or delivery speed within the planned time (Moon et al. 2011)is one of the requirements that the owners and the contractor both expect. Owners supervise and monitor project delivery speed by setting strict milestones or deadlines (Lopez, Mascione, and Liu 2017;Mahdi and Alreshaid 2005) or establishing an incentive mechanism of shortening the time to ensure on-time completion and delay reduction. The contractor's work is relatively specific and complex. The delivery method should have the following functions to achieve the owner's goal, including the time estimation as accurate as possible when signing the contract; incentive mechanisms to ensure the work of the planning, design, pre-construction preparation, construction, procurement stages be shortened  (Alhazmi and McCaffer 2000;Kumaraswamy and Dissanayaka 2001;Mostafavi and Karamouz 2010;Oyetunji and Anderson 2006); reducing interference between the stage or achieving overlap between different phase or work (Alhazmi and McCaffer 2000;Lopez, Mascione, and Liu 2017); a quick feedback mechanism to adjust the follow-up work (Alhazmi and McCaffer 2000). Rapid agreement among the participating entities is an organizational guarantee to achieve a shorter duration of each phase.

Project cost issues
Cost criteria ranked fourth and are mentioned 64 times. The criteria can be divided into cost certainty, cost control, cost requirement/constraint similar to the time criteria. Most owners in regular business projects may seek the minimum costs (Mostafavi and Karamouz 2010;Oyetunji and Anderson 2006), but some just expect the completion within budget (Luu, Ng, and Chen 2005;Marzouk and Elmesteckawi 2015). This cost includes the direct cost based on design and resource limitation, waste, rework, waiting caused by poor communication, and so on due to improper construction management. In a broader perspective, maintenance and operation costs are also included in the cost concept. Choosing the proper delivery method can reduce the uncertainty in design and construction (D. Q. Tran and Molenaar 2014), and reduce the cost caused by waste and idle resources, rework and delay.
The owner needs to establish the project cost target or requirement as early as possible and the cost change and degree that the project may occur Mostafavi and Karamouz 2010). Meanwhile, the owner needs to estimate the precise cost before contract signing Mahdi and Alreshaid 2005), identify the profit from cost savings and encouragement to contractors (Moon et al. 2011), and the less cost of each stage of work (Alhazmi and McCaffer 2000;Noorzai 2020;Touran et al. 2011). Different delivery methods have various cost control means and emphases. DBB method needs to control the cost increase caused by the loss of information transmitted between different phases. DB and CM need to control the management cost of overlapping. The determination of the lowest cost varies according to the degree of integration of design and construction. In practice, value engineering may be an effective tool to optimize costs for both owners and contractors. (Al Khalil 2002;Mafakheri et al. 2007)

Project quality issues
Quality can be considered as the degree to which the designers and contractors can satisfy the owner. The maximization control of aesthetic and physical quality is a significant factor to be considered (Mafakheri et al. 2007). Also, the contractor's reputation, aesthetics, and confidence in design (Ng et al. 2002) will be considered by the owner. Ensuring that the project deliverables meet the owner's needs is the most basic requirement of any delivery method. However, the efforts and measures vary from the organizational differences of delivery methods. In DBB method, each stage is highly specialized, and there is a quality inspection mechanism for mutual supervision between stages, which can ensure the overall quality under ideal conditions. However, because of the stages of separation, participants will focus on improving the quality of the work on their own. It is easy to ignore the quality requirements from the owner and the quality connection between the stages. Quality requirements can be divided into stage quality measures (Alhazmi and McCaffer 2000;Luu, Ng, and Chen 2005

Contract and disputes
Issues related to contracts and disputes are totally counted 44 times. They are different from the physical characteristics of the project and from the characteristics of the owners and contractors, but they are closely related to them. The number and size of contracts in different delivery methods are different Oyetunji and Anderson 2006). In a single contract, a large number of activities convert from external contracts into the internal workflow, which significantly reduces risks and the possibility of disputes. (D. Q. Tran and Molenaar 2014) Multiple contracts are prone to create gaps or overlaps between contracts and arise disputes between stakeholders. However, this does not mean that a delivery method with a few contracts must be better than a delivery method with a large number of contracts. For the owner, the more sub-project means that the project is more fully understood and the scope of work is more clearly defined. For contractors, because of the upstream and downstream workflow constraints, the quality can be more strictly controlled. Whether or not there will be disputes depends more on the clarity of the contract and the completeness of the contract scope Touran et al. 2011). Although the contract seems to be the result of selecting the delivery method, the owner should fully predict the type of contract , evaluate its management capabilities, and in turn make appropriate decisions.

Risk management issues
Risk factors are mentioned 46 times in collected articles and come from all aspects of project implementation, including the scope definition, site, resources, technology, etc. The risks include initial project risk assessment, risk allocation between owners and contractors, and risk management during implementation. The initial risk assessment should not only consider the risks caused by the uncertainties of the natural environment, technology, and resources (J. Y. Ding, Wang, and Hu 2018;Qiang et al. 2015;D. Q. Tran, Molenaar, and Alarcon 2016) but also the risks caused by the discreteness of work and organizational arrangement from the delivery method (Lopez, Mascione, and Liu 2017;Mostafavi and Karamouz 2010;Oyetunji and Anderson 2006;D. Q. Tran, Molenaar, and Alarcon 2016). Appropriate delivery methods can reduce organizational risks. The higher the integration level of the organization, the more accurate and comprehensive judgment of technical risks will be made by all professionals. From the perspective of risk, project delivery means the process of distributing and controlling risks between owners and contractors through organization and work distribution. Theoretically, all risks can be transferred to an organization through the use of specific delivery methods. A perfect design and construction contract may pass almost all the usual risks at a price to a single contractor organization Ng et al. 2002;Touran et al. 2011). Risk management can be conducted by identifying potential changes in construction (Lopez, Mascione, and Liu 2017), minimizing the risk factors (Moon et al. 2011), and transferring risks to more riskcapable participants through contracts (Mostafavi and Karamouz 2010;Oyetunji and Anderson 2006)

External market issues
External market issues include financing and funding, price competition, and market competition status and are mentioned 40 times. No project can exist independently of the external economic environment. In the initial stage of the project, the owner needs to obtain sufficient financing or funding for the project and deal with the risks caused by future price changes (NEDO 1985;D. Q. Tran and Molenaar 2014). It is necessary for the owner to know the accurate price of the project early and how much must pay at the specified time. Early price identification helps the owner to understand the work content and risks, and meanwhile, promotes the contractor to optimize the organization and use of resources (Alhazmi and McCaffer 2000).
Decentralized and sequential implementation work is difficult to achieve early estimation and prediction. Under DB and CM methods, contractors improve the accuracy of early determination through integrated professional collaboration. Although owners and contractors cannot control fluctuating resource prices, proper delivery methods can effectively reduce procurement price fluctuations (Noorzai 2020;D. Q. Tran, Molenaar, and Alarcon 2016). Price competition covers such issues as value for money, maintenance, costs, and competitive tendering (Ng et al. 2002), which cover all stages of the project transaction. The price competitiveness of the delivery method depends mainly on the integration of the work in one stage and between stages. The higher the integration, the more the contractor can convert external costs into internal costs.
External market competitiveness refers to the level of competition in the market regarding this project Luu, Ng, and Chen 2005). The price of the delivery method is affected by the degree of market competition (Luu, Ng, and Chen 2005). The number of potential contractors, the degree of market development, and the market size are common segmentation factors (Demetracopoulou, O'Brien, and Khwaja 2020;Gordon 1994). The highly integrated PDM requires enterprises with strong comprehensive capabilities to complete D. Q. Tran, Molenaar, and Alarcon 2016). Only when the construction market is developed and there are a sufficient number of contractors can there be enterprises with adequate capacity in the market. Under certain conditions, such as post-disaster situations, the construction trading market may shrink due to risks, materials, and project owners cannot find contractors with sufficient capacity to participate in market competition.

Laws, regulations, and policy impacts
Factors about law and regulation are addressed totally 38 times, and political impact are mentioned ten times in previous papers. In almost all project management processes, the influences from laws, regulations, and policies are always the most significant external constraints. The realization of the PDM is restricted, supported, or encouraged by the legal and political framework of the country where the project is located. For instance, the project in U.S. must also comply with federal, state, and local laws (Demetracopoulou, O'Brien, and Khwaja 2020;Touran et al. 2011). Laws and regulations affect the legitimacy of PDMs, which in turn generates project risks. The laws and regulations not only include regulations on the applicability of delivery methods but also include environmental constraints (D. Q. Tran and Molenaar 2014), labor policies (Demetracopoulou, O'Brien, and Khwaja 2020;Touran et al. 2011), andwork standards (D. Q. Tran, Molenaar, andAlarcon 2016). The policy is another issue that owners and contractors need to consider (X. Ding et al. 2014;Qiang et al. 2015). Most of the investors or final beneficiaries of construction projects that involve delivery methods are related to the government. The government expresses its risk propensity and willingness by formulating policies to change project participant behavior.

Project complexity
Complexity indicates whether the owner has a special requirement that requires innovation and a unique construction method NEDO 1985;Ng et al. 2002) and is mentioned 32 times. Project complexity is mainly reflected in its technical and organizational complexity (Jimoh, Oyewobi, and Aliu 2016;Qiang et al. 2015). Complex technical requirements need more professionals and complex organizational structures. Complex projects also require precise contracts to coordinate the behavior of participants. The complexity of the project affects the willingness of the owners and contractors, which in turn affects the choice of delivery methods. Owners of simple projects are usually willing to choose DBB to strengthen monitoring each step of the project. Owners of complex projects may be more willing to choose DB or CM and authorize contractors with stronger capabilities to complete the construction process. Correspondingly, if comprehensive capabilities are sufficient, the contractor will be willing to undertake more complex projects through DB or CM, and on the contrary, the contractor tends to only undertake a certain professional work in the DBB. The project complexity will also reduce the effectiveness of communication between project participants, increase the difficulty of flexible response to changes, and bring more difficulties to the PDM. More complex projects are more prone to unexpected changes, so more structured delivery methods and contract systems should be established at the beginning of the project to reduce the risk of changes.

Flexibility and innovation
Flexibility refers to frequent changes in design and construction once the work has begun on site . The delivery method should be flexible enough to accommodate design changes caused by changes in the environment or requirements and be able to implement design changes quickly during the design and construction phases. (Ng et al. 2002). Flexibility depends on information transmission and feedback path, decision-making mechanism, resource supply mechanism, etc. The information in DBB needs to be transmitted between the contractor, the designer, and the owner, and even more participants when the change occurs. And only when multiple participants have made the decisions, the changes can be implemented. Whereas the DB or CM method usually has only one general contractor and the information only needs to be transmitted within the general contractor and decided by itself. Another function needed is innovation constantly to cope with complex and changeable environments (Demetracopoulou, O'Brien, and Khwaja 2020;Xia et al. 2013). The appropriate delivery method should be unique in terms of contract expression, coordination, and cooperation mechanisms based on the uniqueness of the project. There should be a buffer to encourage designers and contractors to innovate to reconcile design documents and implementation measures (Moon et al. 2011).

Definition of scope
Defining and managing the project scope influence the project's overall success and be mentioned 20 times. Project scope refers to the project's deliverables and works required to create those deliverables (PMI 2013). Defining the work needed to accomplish clearly is an essential condition for determining the method of delivery. The employer needs and has the responsibility to clearly understand the scope of the project work and deliver it accurately to the contractor (Al Khalil 2002;Mahdi and Alreshaid 2005;Noorzai 2020;Xia et al. 2013). The project scope describes the business need, justification, requirements, and current boundaries for the project. The project scope also provides the list of deliverables and acceptance criteria for the project and its products, services, and results. On the other hand, the project scope will inevitably change with the actual situation during project implementation (Mafakheri et al. 2007). In this case, the project scope change should be rapid and accurate. It is relatively easy for owners to determine the project scope for regular commercial projects because there are many similar cases to support, and the external environment does not change dramatically. But for other projects, such as reconstruction projects after a disaster, technical innovation projects, it is more difficult to define the scope accurately. Whether the project owner or contractor, or other project participants need to clarify the specific work of their own participation in the project. Only under such a requirement can participants arrange their resources and methods reasonably and clarify their responsibilities and obligations. In some situations, such as emergency construction, overseas engineering, etc., defining the project's scope may even be fatal to the project's success.

Project size and scale
Project size is not one of the major drivers in the selection process but still may be significant (Mafakheri et al. 2007). The project size can be measured by the number of subprojects or work packages in the project and their size (Gordon 1994) or even its estimated value (Marzouk and Elmesteckawi 2015). A big scale of the project means that more protocols are needed to regulate the work of different professionals and teams and more resources, and more structured tools for project management Liu et al. 2016). DBB method is not suitable for largescale or mega-projects because it has to face a lot of work packages from design, bidding, construction, and professional subcontracting. A large number of external agreements between work packages require appropriate and efficient organizational coordination mechanisms.

Public participation and culture
Although public participation and cultural factors are not the primary factors and are mentioned 19 times and six times, respectively, they are unavoidable factors to almost all projects. The project is always in the region's public participation and cultural constraints, where this cultural constraint runs throughout the project. Objection from a neighbor or local lobby group to construct the project (Marzouk and Elmesteckawi 2015) will terminate the project from the beginning. The bias toward bidding could impose impacts on the selection process (Mafakheri et al. 2007). For instance, resources like raw materials and workers in construction cannot be sufficient without the local company or labor involvement. The relationship between the contractor and local community or stakeholders may hinder the project acceptance when the project closed.

Resources supply
The effective supply of resources, including but not limited to materials and quality of resources, are considered to be related to the decision to choose the delivery method. Materials availability means the availability of materials as required in projects specifications (Luu, Ng, and Chen 2005;Marzouk and Elmesteckawi 2015). The market and purchasing process of resources are two aspects to consider when analyzing the availability of resources. Smallscale and regional material markets may hinder or weaken the willingness of foreign competent DB or CM contractors to enter and force owners to choose more decentralized DBB methods. The procurement time, procurement entity, procurement price, information, and other issues in the procurement process directly or indirectly generate project risks and choose different PDMs. Once the exact admission time of resources cannot be determined, the risk of project implementation increases. The owner should consider this risk and choose a delivery method with maximum flexibility. If there are multiple procurement entities, it may lead to differences in procurement standards, procurement processes, and procurement prices of materials and produce a series of impacts in the process of transferring materials among participants. Although some owners may withdraw their rights to purchase materials, there is still a large number of scattered material distribution processes, which will cause uncertainty in construction arrangements and confusion in the work of the implementation team.

Project site
Site issues also are mentioned 19 times by researchers to be a factor in choosing the delivery method. Site constraints may affect the task allocation for design and construction Luu, Ng, and Chen 2003). Site constraints may affect the assignment of design and construction tasks. When there are more restrictions on the site, the owner needs to consider whether to provide fewer design documents to allow more space for the contractor to develop creativity and innovation or provide more documents to describe the adverse effects they may encounter (J. Y. Ding, Wang, and Hu 2018;Xia et al. 2013). In addition, when site uncertainty increases, owners should provide more scope definitions or more information about uncertainty and share their understanding and knowledge with designers and contractors. By describing site constraints more clearly, the owner can provide fewer or no design documents, allowing designers or contractors to find the best solution more autonomously.

Project type
Project type is mentioned only 13 times, but in the delivery method choosing, the process is often used as the first step . Construction projects have industrial, infrastructure and buildings, and other different types Liu et al. 2016). Each of them has its own characteristics, which leads to significant differences in project management methods and technical arrangements. Different types of projects require different delivery methods criteria, as Chen et al. believe that "delivery speed" is mainly used for building projects, but not for other types of projects. There is no comparability between different project types ).

Other project-related issues
Natural disasters and sustainability are the last two factors mentioned by researchers a total of 17 times.
Natural disasters criteria refer to the probability of natural disasters that might hinder the project activities (Marzouk and Elmesteckawi 2015). The organization in any delivery method is always temporary. If work is interrupted due to a high probability of disaster, the stability of the organization will be challenged. For owners and contractors, it is crucial to choose a suitable delivery method to maintain the stability of the entire organization, which can reduce the repetitive work and waste caused by the replacement of contractors or subcontractors.
Sustainability is not a traditional but an emerging project objective. Most of the previous studies of sustainability of construction focus on recycling and waste reduction or sustainable construction method. As a factor to select the delivery method, it is a new area. A survey conducted by Touran in 2011 showed that most agencies interviewed had not used these concepts in the past, but it was felt that these factors would have to be considered in future projects . Marzouk and Elmesteckawi (2015) described sustainability as the requirements to achieve the anticipated results of the green revolution manifested in reductions in energy consumption, better health, and higher productivity for occupants. Sustainability in construction includes sustainable design and sustainable construction. The PDM integrates the design with construction and other processes to varying degrees, and the appropriate delivery method can transform the staged sustainability into the sustainability of the entire process.

Discussion
Twenty-two factors are revealed to impact the selection of PDM, indicating the complexity of the decision process. However, this list can only provide decisionmakers with a comprehensive project review or auxiliary decision-making tool. When facing actual construction projects, it is necessary to select more comprehensively from the deep-seated issues among these factors. These issues are also the points for future research.

Interaction between factors
The factors do not exist independently but are related, and the interaction may be positive or negative. For example, Hosseini et al. (2016) believe that contractor's capability will be influenced by cost and time certainty, risk allocation, and quality performance. At the same time, the quality performance will be influenced by the contractor's capability, technological availability, complexity, and innovation. Luu, Ng, and Chen (2005) also observe that the potential influence between factors may exist within projects. This means that the method of establishing project delivery choices must consider the established standards and the priority changes brought about by the interaction between the standards. Just like the triangular relationship between the three elements of traditional project management, excessive attention to one element will bring about negative changes in the corresponding performance of other elements. For example, emphasizing quality will increase costs and time. The understanding of interaction increases the choice of effectively promoting standards-based delivery methods but at the same time increases the difficulty of the allocation of resources, capabilities, and organizations for all parties.
However, most research on the choice of delivery method will still not be able to consider the interaction between factors fully. (Al Khalil 2002;Mafakheri et al. 2007); Mahdi and Alreshaid (2005); (Oyetunji and Anderson 2006) used AHP or its improved methods, but these methods are not effective in analyzing the interaction between factors at the same level. It may be an appropriate research direction to carefully consider standard interactions from a system perspective to determine the best balance.

The number, weight, and level of factors
This article ranks the factors according to the frequency of the researcher's attention. However, the frequency mentioned by the researchers does not represent the importance of the factor when choosing PDM, nor does it mean that the actual project needs to be used all, but only indicates the degree of recognition imputed by them. When determining the delivery method, the decision-maker should take the following steps to these factors: a) determines the priority of the factors; b) determines the required factors; c) determines the weight of each factor, and d) determines the interaction between the factors. For example, the project type has only been mentioned 13 times, but it is one of the highest priority factors determining the delivery method and even in turn affects the ultimate performance of the project (Luu, Ng, and Chen 2003). Only after the project type is determined can the indicators suitable for the project be chosen, and then the owner can judge the factor's importance. In certain types of projects, some standards may be considered the main standards, while others may be completely ignored. Large and complex infrastructures may require more consideration of the contractor's capabilities and flexibility to handle changes in delivery methods. Emergency construction projects have stricter time limits. In the post-disaster reconstruction projects, when various departments reach a consensus, political influence can be ignored. In addition, public and community participation may also play an important role.
For the vast majority of conventional construction projects, the first few items of statistical results become the priority for selecting PDM. At the same time, some of the statistical influencing factors have little influence on PDM of general commercial projects, which will hardly be considered during project decision-making and construction, such as natural disasters, political influence, culture, etc. After detailed geological investigation and comparison, the site selection of most construction projects is extremely unlikely to encounter natural disasters during the construction period, so there is no need to avoid risks through PDM selection. However, with the increasing complexity of projects and the increasing number of projects in specific environments, some early criteria that initially only a few researchers recognized may gradually become mainstream, such as design sustainability and construction sustainability. This shows that the selection criteria and weights of each project need to be redistributed according to project characteristics, owner characteristics, and external environment. The PDM selection process is a complex task, and specific project goals and conditions can play an important role. The unique characteristics of each project should be evaluated to determine which PDM can produce the best results (Dai Q Tran et al. 2013). In other words, the owner should determine the project needs, consider the special circumstances of the project, and choose the best method to achieve acceptable results.

The quantification of factors
The quantification of criteria is still an important research area. Among the 22 criteria and 35 sub-criteria that are identified, only a few of these can be quantified, notably time and cost. The majority of index factors remain fuzzy qualitative criteria. In practice, it is difficult to make appropriate choices precisely or immediately. Making choices within a limited time window is, in some contexts, such as in reconstruction after a disaster, precisely the most critical. How then to accurately reflect fuzzy criteria in practice is still a critical problem that needs further attention. Professional investigation and subjective professional judgment are currently the main methods of criteria identification and assessment. Previous studies have conducted a range of research through AHP(Al Khalil 2002), TOPSIS (Mostafavi and Karamouz 2010), and the triangular fuzzy number method. However, these methods are still based on the modification of the personal preferences of experts, and the results obtained still do not objectively reflect the actual project, making it difficult to find any correlation between factors. It is thus necessary to establish a recognition method that excludes subjective judgment to the extent possible. The processbased modeling method seems to be a feasible approach. This involves identifying the relevant factors by plotting various activities and required functions in the planning, design, construction and other processes of the various delivery methods. The element requirements based on the objective activity function requirements appear to minimize subjective judgment.

More detailed project scenarios
Each project needs to identify its own suitable selection criteria and then settle on the most appropriate PDM. The diversification of research perspectives can enrich the criteria and the knowledge domain of PDMs. As mentioned, the project category is an important selection factor. The considerations regarding different sorts of projects may differ; however, there are few studies that shed light on the requirements of different types of projects, with some exceptions: (Demetracopoulou, O'Brien, and Khwaja 2020 Subsequent research should continue to investigate the project delivery requirements of specific types of construction projects, such as public buildings, bridges, water utilities, energy, and other types of projects. A further area that needs to be investigated is unconventional construction. In some special situations, such as post-disaster reconstruction, most project constraints and conditions, such as cost and time constraints, owner needs, and external market conditions, are largely disrupted from the norm. Here, the emergency will greatly skew the number, weight, and mutual influence of delivery method criteria. Decisionmakers should consider some changes in requirements, such as completing the project more quickly without reducing quality as the primary consideration; whether the project delivery method is flexible enough to deal with various emergencies in construction.

The influence of factors on the performance of PDM
The research of the factors in the list that influence the performance of the PDM can help understand and make the decision. The research on the influence mechanism of factors on performance will help decision-makers decide whether to adopt the factors and their weights. Some existing research focuses on some of the more critical factors in the list. For example, Liu et al. discussed the influence of owner characteristics Xia et al. 2013). Others  and ) discuss the characteristics of the project and the impact of the external environment separately. However, it seems that the influence and mechanism of some factors that have not received traditional attention should also be studied in depth. For example, the project execution organization often must consider the opinions of the community when working. In the highly integrated project delivery method, the professional departments of the project can quickly, accurately, and comprehensively share these opinions, reduce the deviation and delay caused by the transmission of information, and improve the project's efficiency. However, how to effectively transmit this information to various professional departments and adjust project organization behaviors were not discussed in depth. An in-depth and detailed discussion of the factors affecting project delivery methods and mechanisms will help reflect the differences in PDM choices more fully.

Limitations and Recommendations for Future Research
This study attempts to establish an auxiliary tool to understand PDM selection factors, so only statistical analysis of PDM selection factors in mainstream literature is carried out. After establishing a list, decision makers in new fields and new types of projects can extract factors suitable for the project, thereby simplifying the PDM decision-making process and reducing errors caused by improper PDM. At the same time, it is an initial effort of a series of studies on project delivery in a specific environment, including selecting project delivery methods under complex conditions such as emergency conditions and post-disaster reconstruction.
Also, the research provides decision-makers with another perspective; that is, appropriate PDM should be designed based on these factors instead of choosing from existing PDMs. As the complexity of the project increases, the existing PDM contract structure may be incompatible with the project operation process. Therefore, designing rather than choosing PDM may be a better alternative.
Some issues need to be discussed in-depth in future research, including exploring the relationship between elements, developing new decision-making methods, establishing suitable selection models, and discussing the mechanism and transmission path of PDM and project performance.

Conclusion
No single Project Delivery System is appropriate for all projects (Gordon 1994;Love et al. 1998;Miller et al. 2000;Ibbs et al. 2003;Gransberg et al. 2006;Touran et al. 2011;Tran et al. 2013;Tran and Molenaar 2015). Similarly, there is no best delivery method, only the most suitable delivery method. Proper PDMs will directly affect the effectiveness of the organizational arrangements and resource allocation of owners and contractors, which in turn will affect the success of the project. Therefore, it is necessary to carefully select the appropriate PDM at the outset of the project. The PDM needs to determine the selection criteria for the delivery method appropriately. Not all selection factors are suitable for all projects, but establishing a list of selection criteria for PDMs helps understand the project status more clearly, and optimize the selection process. This paper provides a detailed review of the factors considered in the selection of PDMs. Through a literature review, a series of general factors are summarized, including time, cost, quality, risk, project scope, owner's issues, contractor's issues, resource availability and quality, external market factors, and corresponding sub-criteria. Future research can select and empower common criteria for specific projects and environments. In addition, the quantification of criteria still needs further investigation. Determining the criteria that affect PDM selection in different project types and scenarios will facilitate optimizing PDM selection, which is expected to increase the success rate of project delivery.