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JSSMH, when human activity is involved in job processing, the job processing time can be affected by
variability of human manipulation, such as random redundant motion or slowing down due to tiredness
(Fink et al., 2014; Liu, Fan, Zhao, & Wang, 2017). Jobs themselves can have inherent variability in
processing time too. For example, metal products’ operation time can be influenced by a series of factors
(Yang, Chen, Wei, & Chen, 2018), as well as industrial chemical processes (Bonfill, Espuna, &
Puigjaner, 2005). There are two common types of variation reported in the body of literature, processing
time in random distribution and deteriorating processing time. However, variable processing time has not
been considered in job shop scheduling when material handling is part of decision making. With material
handling system as an integral part of production, it is essential to take this into consideration when
making production decisions.
Random processing time in production scheduling problems usually results from inaccurate data
collection or uncontrollable operations. Sakawa and Kubota (2000) applied genetic algorithms to fuzzy
programming for multi-objective job shop scheduling problems in which uncertain processing time and
due date were introduced, and in the case study each operation had three possible realized processing
times in triangular distribution. Bonfill, Espuna, & Puigjaner (2005) formulated a two-stage stochastic
programming model based on job shop scheduling for chemical processes where reaction time is
uniformed distributed. Such models were also described as Stochastic Job Shop Scheduling (SJSS)
problems, while the material handling was not included and it was often assumed that operations could
start immediately after completion of the previous operation. In reality, introducing material handling to
the optimized solution of SJSS will make the problem more realistic; however, also much more
complicated. Hence simulation has been commonly utilized when randomness exists in JSSMH (Xie &
Allen, 2015). With a large number of experiments, simulation could help in developing heuristic shop
floor management strategy (Wang et al., 2014). The strategy can also be flexible to implement operation
mechanisms, such as behavior rules (Ng, Eheart, Cai, & Braden, 2011; Y. Zhang, Huang, Sun, & Yang,
2014) and optimization-based decision making (Almeder, Preusser, & Hartl, 2009; Sacone & Siri, 2009).