# CiteSpace-Based Bibliometric Review of Pickup and Delivery Problem from 1995 to 2021
###### Abstract
Bibliometric analysis is a methodology that quantitatively performs literature citations and content analyses for a particular research topic, as well as visualizes the research status quo and development trend based on bibliographic elements such as author, keyword, journal, country, institution, and reference [11; 12; 13]. [PERSON] and [PERSON] [14] examined the cited reference distribution of 30,051 documents published in Spanish regional journals, and further found that there indeed exists a previously undiscovered citation network unique to these regional journals, which can be well served as an indicator for the future academic evaluation of Spanish journals. Up to now, bibliometric analysis has been applied to diversified professional areas to conduct a panoramic representation, and helped interested scholars to carry out follow-up research with a target in mind. In recent years, some assistive software packages were developed to make bibliometric analysis more user-friendly, e.g., CiteSpace, RefViz, and HistCite. As shown in Table 1, compared with RefViz and HistCite, CiteSpace provides more powerful functions, various scientific knowledge map, and complete parameter settings to match the structure and characteristic of different fields (e.g., [15; 16; 17; 18; 19]). Given these, this paper conducts a bibliometric analysis of PDP research with the help of CiteSpace software package, and aims to answer the following specific questions:  
Q1: What is the posting trend in terms of the quantity of publications? Q2: Which authors, journals, countries, categories, institutions, and documents compose the main force in the research on PDP? How do they connect with others? Q3: What are the current research hotspots and research gaps? Which research directions are worthy of further study? Q4: What has been the evolution trend of PDP research since it was initially proposed? What are the distinct characteristics during each development stage?To be specific, the main innovations and contributions of this paper are summarized as follows:  
1. We first adopt CiteSpace to visualize and analyze the scientometric characteristics and relationships among PDP-related publications sourced from Web of Science. 2. Based on the quantitative statistical results of bibliographic elements (e.g., authors, journals, keywords, countries, institutions, and references), we illustrate various scientific knowledge maps and corresponding tables to elaborate the current status of PDP research from different perspectives. It is noted that the results generated by CiteSpace are more intuitionistic and impartial than manually produced results. 3. None of the reviews regarding PDP research discussed how the research hotspots and knowledge domains evolve over time. We first attempt to highlight the hot topics of PDP research during different development stage through keyword citation burst detection, and recognize the chronological distribution of knowledge domains through reference co-citation clustering. 4. According to the aforementioned findings, insightful recommendations are provided to guide enthusiastic scholars and logistics practitioners to pursue new and promising avenues of future research, which have rarely been considered in previous studies. The remainder of this paper is organized as follows: In Section 2, we briefly review the research method of CiteSpace after introducing the procedures of data collection and processing. In Section 3, the basic statistical results sourced from Web of Science are displayed. In Section 4, we use CiteSpace to generate a series of scientific knowledge maps, and elaborate the corresponding results of bibliometric analysis and visualization analysis. In Section 5, we make an in-depth discussion of evolution trend of PDP research, judging from the development history of research hotspot and knowledge base together. Besides, conclusions and outlooks are listed in Sections 6 and 7, respectively, followed by the limitations in Section 8.