# A Survey of the Literature on Order-Picking Systems by Combining Planning Problems
## 5 Discussion
### Investigating the Problems of Order-Picking Planning and Their Combination
#### 5.1.2 Study of the Components of the Order-Picking Process
In the current paper, the most studied areas were routing and storage space since a total of 21 articles studied warehousing and the design of a picking warehouse to reduce or optimize routes [1; 24; 9; 36; 40; 41; 42; 48; 49; 50; 51; 52; 61; 62; 64; 70; 71; 72; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82]. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Auditors** & **Assignment** & **Routing** & **Batching** & **Sequencing** & **Tardiness** & **SUM** \\ \hline [PERSON] et al. [30], [PERSON] et al. [1], [PERSON] \& [PERSON] [64], & & & & & & \\ [PERSON] et al. [3], [PERSON] \& [PERSON] [83], [PERSON] \& [PERSON] [34], [PERSON] \& & & & & \\ [PERSON] [44], [PERSON] et al. [49], [PERSON] [53], [PERSON] et al. [45], [PERSON] et al. [43], [PERSON] \& [PERSON] [41], [PERSON] \& & & & & \\ [PERSON], [47], [PERSON] et al. [70] & & & & & & \\ [PERSON] [14], [PERSON] et al. [38] & & & & & & \\ [PERSON] et al. [20] & & & & & & \\ [PERSON] et al. [46] & & & & & & \\ [PERSON] \& [PERSON] [55], [PERSON] et al. [84], [PERSON] \& [PERSON] [33], [PERSON] \& [PERSON] [85], [PERSON] et al. [52], [PERSON] \& [PERSON] [86], [PERSON] et al. [87], [PERSON] et al. [5], [PERSON] [37], [PERSON] \& & & & \\ [PERSON] [23] & & & & & & \\ [PERSON] \& [PERSON] [56], [PERSON] \& [PERSON] [40], [PERSON] et al. & & & & \\
[61], [PERSON] et al. [50], [PERSON] \& [PERSON] [34], [PERSON] et al. [31], [PERSON] et al. [85], [PERSON] et al. [60], [PERSON] et al. [78], [PERSON] et al. [7], [PERSON] \& [PERSON] [17], [PERSON] et al. [54], [PERSON] [35], [PERSON] \& [PERSON] [16], [PERSON] [79], [PERSON] et al. [89], [PERSON] et al. [90], [PERSON] et al. [91], [PERSON] \& [PERSON] [51], [PERSON] \& [PERSON] [80], [PERSON] et al. [42], [PERSON] \& [PERSON] [92], [PERSON] \& [PERSON] [15], [PERSON] et al. [29], [PERSON] et al. [81], [PERSON] \& [PERSON] [82] & \\ [PERSON] \& [PERSON] [59], [PERSON] \& [PERSON] [60], [PERSON] \& [PERSON] [93], & & & \\ [PERSON] et al. [25] & & & & & & \\ [PERSON] et al. [21] & & & & & & \\ [PERSON] et al. [12], [PERSON] et al. [62], [PERSON] et al. [71], [PERSON] et al. [67] & & & & & \\ [PERSON] et al. [36] & & & & & & \\ [PERSON] et al. [19], [PERSON] et al. [39] & & & & & & \\ [PERSON] et al. [24], [PERSON] et al. [22], [PERSON] \& [PERSON] [94] & & & & & \\ [PERSON] et al. [18] & & & & & & \\ [PERSON] et al. [13] & & & & & & \\ [PERSON] [95] & & & & & & \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Areas of Study in the Picking Process. Moreover, to find the most economical routes, warehouse layout, piecewise assignment, and parallel scheduling of equipment pickings, travel routes have been studied in all logistic processes and are mainly the most influential factors in terms of costing and timing. In [PERSON] et al. (2020), mathematical programming modeling was considered in the above categories for the first time. This research evaluated order batching and picker routing in OPS under the title of \"the benefits of integrated routing.\" [PERSON] and [PERSON] (2019) stated that the order batching problem involves grouping a set of customized picking orders to minimize the entire picker journey so that the picking will be considered based on the calculation of the picking trip length. The strategy of this research was to simplify the order-picking activity by simplifying the custom picker journey. [PERSON] et al. (2019) introduced a mathematical programming model for the order batching and routing problem, as well as a model for the batching, sequencing, and routing issue. For this purpose, they provided some formulas for the distance traveled and the pick time in the picking process in single-block and multi-block warehouses. Moreover, [PERSON] et al. (2019) focused on human communication and ergonomic parameters in OPS by noting that most warehouse activities are manually performed by human operators. This paper also emphasized that performance and cost are profoundly dependent on availability and productivity affected by operator fatigue.