# 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  
Investigating the assignment and sequencing of the batch to multi-pickers concurrently with the optimal routing problem, checking the overall time of the custom multi-picker picking process, or applying a categorized warehousing method are generally done to maximize profit. In Figure 7 and Table 3, components of the order-picking process can be observed. As shown, it tries to optimize order assignment (sorting and routing of the shippers resulting in minimizing process time). In addition, the workload balanced between the shippers is maintained. There were 27 articles that considered cost (Table 3) [6, 13, 15, 16, 18, 21, 30, 36, 41, 42, 50, 51, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69]. Most of the cost-related studies focused on changes in the warehouse system, reductions in routes, improvements of the assignment process, changes in the shape and size of warehouses, travel times, and changes in warehouse design processes. This group of studies provided methods for solving order-picking problems to help managers make better decisions. They also considered some of the most critical aspects of order-picking for identifying scheduling problems and issues with different topics. Further, some research works investigated order-picking other than resources [13, 14, 20, 22, 33, 36, 70, 71]. Only the study by [PERSON] et al. [20] on tardiness simultaneously investigated five critical  
Figure 6: Order-picking process aspects ([PERSON] et al. [2]). \begin{table}
\begin{tabular}{c c} \hline \hline
**Performance Index** & **Articles** \\ \hline Cost & 27 \\ Equipment & 9 \\ Storage space & 37 \\ Assignment & 8 \\ Routing & 54 \\ Batching & 43 \\ Sequencing & 14 \\ Tardiness & 6 \\ Ergonomy & 11 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The Picking Process Components Study Section. Figure 7: Frequency of study areas examined in this study. aspects of the order-picking process (assignment, routing, batching, sequencing, and tardiness). Four more studies [12; 60; 67; 72] also simultaneously studied routing, batching and assignment. Table 4 provides an overview of the combination of order-picking scheduling problems about various aspects of its implementation. Five main aspects defined in the order-picking process were included in 73 studies on implementing the combined planning process problem. Nineteen studies did not study these five categories and solely focused on ergonomics, cost, equipment, and storage space [2; 4; 6; 9; 28; 32; 56; 57; 63; 65; 66; 68; 69; 72; 73; 74; 75; 76]. Therefore, we can state that only 20% of the reviewed studies did not focus on the five main categories of research, indicating the importance of assignment, routing, batching, sequencing, and tardiness in the order-picking process. According to Table 4, the most significant combination of routing and batching studies with 15 cases and routing with 26 cases reflects the fact that researchers have focused on routing. [PERSON] et al. [62] used both simulation solutions to simultaneously examine picking processes to reduce operating costs and a hybrid algorithm to generate batches and assignment and sequencing customer needs. Several studies studied scheduling problems together in a multi-block warehouse, while combining batching and routing into a single scheme. After assigning the order to the packer, any order picker can begin the journey to the warehouse to select and pick. The orders should be prioritized according to the production plan and delivered to the pickings. In the meantime, the lost time is one of the most important factors to consider to prevent tardiness and increase productivity. For example, when a picker stops to prepare an order batch, it is one of the cases of tardiness in the order-picking process. The studies reviewed in [PERSON] et al. [2] addressed the reduction or elimination of tardiness through redesigning the picking process, batch organization, routing, prioritization, and picking activities.