# A Survey of the Literature on Order-Picking Systems by Combining Planning Problems
## 6 Results
In this section, the results of research questions are discussed. Figure 2 and Table 4 present an overview of the approach implemented in most articles intended to solve any combination of picking planning problems, as well as the number of articles that analyze each combination. Regarding the first research question, Figure 7 and Table 3 show that in the order-picking area, a combination of planning problems, such as cost, equipment, warehousing, assignment, routing, batching, sequencing, and tardiness were considered. Regarding the second research question, it was found that the most studied issue (54 articles) was related to routing, and in comparison, tardiness received the least attention with 6 articles. This indicates that researchers' tendency is to study the issue of routing compared to other aspects of picking operations (Figure 8). Through investigating the order-picking problems in the articles (based on Table 4), it was found that most of the studies were on routing with 26 works and batching and routing with 15 cases. This reflected the fact that the research base in the order-picking area is routing. One of the important points in the picking process improvement is the integrated review of all aspects of the picking process. For example, four studies simultaneously surveyed the categories of assignment, routing, and batching [12; 61; 67; 71], and one study simultaneously surveyed the categories of assignment, batching, routing, sequencing, and tardiness [20] (see Table 4). As can be observed in Table 7, most studies focused on batching and routing to increase productivity. However, [PERSON] et al. [67] explored a concurrent survey on assignment, batching, and routing, and [PERSON] et al. [22] simultaneously studied routing, batching, and sequencing to maximize productivity in OPS. Different combinations of picking problems were utilized to improve storage space (Table 8) and minimize the makespan (Table 9). In addition, according to the fields of study, the articles reviewed in this work can be approached in terms of either tactic or component. According to the research method of the present study, the selection of valid search engines can help to review the literature appropriate to the area of research. In the reviewed literature, three types of articles were observed: articles presenting new approaches and methods in the picking process, review articles, and articles focusing on the components of the order-picking process. The classification of these studies helps warehouse managers differentiate between planning problems, become aware of the minimum combinations used, and detect which planning problems should be simultaneously considered. Considering the third research question of the present study, Figure 5 shows that one of the main uses of a cross-dock warehouse is to route the picker in cross aisles. Therefore, it is necessary to assign routes to reduce picker travel in a warehouse and decrease the number of visits to the block bypassing the aisle. In the previous studies, a classification of the warehouses can be seen. In Table 2, it is obvious that parallel-aisle warehouses were the most studied warehouses. in overall performance. In this paper, studies that highlighted the main issues of the picking process were classified through reviewing the literature. By focusing on planning problems that affect an economic goal, such as time-related performance or productivity indicators, problems were investigated as these goals are the most important warehouse operations. In this paper, regarding a combination of order-picking planning problems, articles reviewed by analyzing different tactical and operational planning problems simultaneously were classified with the aim of determining which planning problems are related, how various individual planning issues are related, and how can warehouse managers take advantage of the combination of multi-objective planning problems to cope with new market developments.