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This section presents a categorization of publications in cross-training with consideration of possible strategies. Decision-making about cross-training strategy identifies which workers should be cross-trained in how many and for what tasks. Reviewing the literature revealed that there are three general strategies used: partial cross-training, full cross-training, and cross-trained teams (see Table 2-3).

No cross-training is the traditional way of working in which no worker performs more than one task. In the cross-training literature, it is usually used as a benchmark to help measure the benefits of other cross-training strategies (Burleson et al. 1998; Arashpour et al. 2015).

In partial cross-training, the workforce is cross-trained to undertake a few tasks in addition to their primary task. In this strategy, full cross-training is avoided due to a large number of aggregated tasks required to be completed or because of specific human resource policies (Burleson et al. 1998).

The configuration of partial cross-training in a project context is a controversial matter and is mainly dependent upon one-off characteristics of projects and the purpose of cross-training (Haas et al. 2001). The structure of cross-training could be unique and the number of tasks performed by a multiskilled workforce could range from two (Burleson et al. 1998) up to eight (Hyatt et al. 2004). However, there are some common configurations in manufacturing and service contexts (Hopp and Oyen 2004; Aksin et al. 2007). For example, Qin et al. (2015) categorized chaining and

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direct capacity balancing as famous forms of partial cross-training in operational research, which are also applicable to prefabricated construction (Arashpour et al. 2015).

Table 2-3. Multiskilling strategies

Reference Cross-training strategy NC Partial Cross-training FC CT DS CH UP DO CC DCB FSH FS Burleson et al. (1998)        Hegazy et al. (2000)     Ballard (2001)  Haas et al. (2001)     Tam et al. (2001)   Gomar et al. (2002)   Koch (2002) 

Koch and Marton (2002) 

Hyatt et al. (2004)  

Thomas and Horman (2006) 

de Miranda Filho et al. (2007) 

Sacks and Goldin (2007) 

Ejohwomu et al. (2008)    

Lill (2008) 

Lill (2009) 

Wang et al. (2009)    

Hyari et al. (2010)  

Liu and Wang (2011)  

Wongwai and Malaikrisanachalee (2011)  

Liu and Wang (2012)  

Arashpour et al. (2015)          

Pandey and Maheswari (2015)  

Ahmadian Fard Fini et al. (2016a)   

Ho (2016)  

Arashpour et al. (2017)          

Gouda et al. (2017)   

NC= No cross-training, DS= Dual-skill, CH= Chaining, UP= Upstream, DO= Downstream, CC=Customized cross-training, DCB= Direct capacity balancing, FSH= Four-skills-helpers, FS= Four-skills, FC= Full cross-training, CT= Cross-trained teams

In repetitive construction projects, there are studies which suggest a combination of two skills (Pandey and Maheswari 2015). However, other studies showcase the potential for incorporating extra skills or even full multiskilling due to common features which are shared between different crafts, which makes learning cost and time effective (Ahmadian Fard Fini et al. 2016). Accordingly, Liu and Wang (2012), Liu and Wang (2011), Tam et al. (2001), Hyari et al. (2010),

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and Wongwai and Malaikrisanachalee (2011) integrated two or three skills depending on the situation.

Hegazy et al. (2000) incorporated two different multiskilling strategies. In the first multiskilling configuration, there is just one type of multiskilled workforce capable of performing one additional task. In another strategy, there are five substitution rules for multiskilling. Some workers are specialists and others are capable of performing two or three tasks.

Chaining is a cross-training strategy in which workers are enabled to undertake operations in their original workstation and also subsequent workstation. It is considered as an appropriate cross- training strategy when there is a functional proximity between different job locations and the level of work in progress is low (Qin et al. 2015). This cross-training configuration was initiated in manufacturing (Hopp and Oyen 2004) and then developed in off-site prefabrication contexts (Arashpour et al. 2015; Arashpour et al. 2017).

A dual-skilling strategy aims to enhance the performance of workers in projects based on identifying complementary workloads (Burleson et al. 1998). For example, an idle ironworker in a project can assist overloaded painters. The main complexity with this specific type of cross- training strategy is that skill demands are project specific and also significantly diverse. Therefore, the structure of this strategy varies in different projects.

A four-skills labor strategy is suggested by Burleson et al. (1998) and is based upon the principle that craft workers can be categorized into four general classifications including civil/structural, general support, mechanical and electrical, reflecting different stages in a construction project. Four-skills-helpers strategy has the same structure as four-skills strategy, however, training is limited to helper level.

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Full cross-training means every worker can perform all required tasks. Obviously, this strategy provides the most flexibility; however, the corresponding costs of training, salary and transfer expenditures are high (Burleson et al., 1998). Although this strategy, is sometimes applicable to off-site construction, it should be noted that in the literature the number of workstations incorporating a full cross-trained workforce usually does not exceed four or five (Daniels et al. 2004). This reflects cross-training costs and potential learning and forgetting effects.

There is no study investigating full cross-training in construction projects as the number of extra tasks is significant (Ejohwomu et al. 2008). Burleson et al. (1998) recognized this strategy as economically inefficient and impractical in the construction project context. However, in theory, this strategy can be used as an upper bound to evaluate related benefits achieved by other multiskilling strategies. Gomar et al. (2002) showed, the benefits of multiskilling become marginal after obtaining competency in two or three trades.

The other area allowing application of full cross-training is repetitive construction projects because of the limited number of tasks and similarity between different jobs. This reduces the time and cost for the workforce to master other skills and also alleviates learning and forgetting effects (Ahmadian Fard Fini et al. 2016). In this context, Gouda et al. (2017) tested the applicability of full cross-training in a pipeline construction with nine repetitive activities.

Cross-trained teams can be formed by grouping a number of multiskilled individuals or by combining differently skilled individuals (Koch 2002). Size of the teams (Thomas and Horman 2006), similarity of skills (Koch and Marton 2002) and ability of working in parallel (Wang et al. 2009) are important factors in configuring multiskilled teams.

Multiskilled teams attract some attention in the site construction context (Koch, 2002; Koch and Marton 2002; Sacks and Goldin 2007) and in repetitive construction (Thomas and Horman 2006).

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However, this strategy is absent in off-site construction contexts and this provides opportunity for future research. Sacks and Goldin (2007) proposed the use of multiskilled teams along with other initiatives to minimize production makespan. The benefit of using multiskilled teams arises mainly from having an increased number of work teams that can operate in parallel instead of working in series. This is analogous to having two different machines in a production line working in parallel to reduce queuing time (Sacks and Goldin 2007). Thomas and Horman (2006) measured how using multiskilled teams influence performance measures in concrete work in a residential building and bridge construction.