# CiteSpace-Based Bibliometric Review of Pickup and Delivery Problem from 1995 to 2021
###### Abstract  
In this paper, we adopt the bibliometric analysis software CiteSpace to analyze the research status quo and evolution trend of pickup and delivery problem (PDP), an important real-world issue occurring in logistics and transportation. We obtain 819 documents with the topic of PDP that were published in the Web of Science core collection during the period 1995-2021, and acquire their basic situation of posting trend and category distribution. Next, we employ CiteSpace to draw scientific knowledge maps and perform the corresponding visualization analysis, which mainly include the following aspects: (a) collaboration analysis of author, country, and institution; (b) co-citation analysis of author, journal, and reference; (c) citation burst detection of keyword; (d) co-citation clustering analysis of reference. The results show that PDP research has gradually become interdisciplinary and highly comprehensive, and the evolution trend of hot topics also reflects that the research directions involve multiple academic disciplines and professional areas ranging from algorithm design to logistics management. The changing knowledge components reveal the fact that the development of PDP research is highly related to the diversity and uncertainty of realistic logistics industry contexts. Study in this paper provides comprehensive understandings of PDP research for scholars and logistics practitioners, inspiring its further investigation. bib12 2022 29 April 2022 29 April 2022 29 April 2022 29 April 2022 29 May 2022  
static pickup and delivery problems (SPDPs), and divided all collection of SPDPs into three categories in terms of different origin-destination structure and the quantity of commodities, i.e., many-to-many problems, one-to-many-to-one problems, and one-to-one problems. Later, [PERSON] et al. [6] also presented some valuable general concepts to the dynamic pickup and delivery problems (DPDPs), and reviewed the DPDPs-related competitive analysis and heuristics that always serve as important tools for addressing practical engineering problems. [PERSON] et al. [7] outlined the PDPs arising in the transportation of goods from many-to-many, one-to-many-to-one, and one-to-one perspectives, and briefly presented the solution methods applicable to the aforementioned three situations, which includes exact and heuristic algorithms. [PERSON] and [PERSON] [8] summarized mathematical formulations and solution methods of a variant of the PDP with time windows (PDPTW), also known as dial-a-ride problem (DARP) when the transportation requests are sent by passengers. Besides, they discussed subproblems of DARP with regard to people movement, including school bus routing and carpooling. [PERSON] and [PERSON] [9] analyzed the latest mathematical models and solution methods applied in the field of PDP, and emphasized the necessity that variables associating with the environment should be incorporated into the future research framework. [PERSON] et al. [10] recently presented a detailed review of vehicle routing problem with simultaneous pickup and delivery (VRPSPD) from five dimensions, namely, mathematical models, solution algorithms, extensions, variants, and case studies. Although the reviews mentioned above provide very valuable insights into PDP-related literatures and achievements, such studies are relatively scattered and largely based on personal knowledge and understanding, leading to a clear and comprehensive picture of the overall PDP research is still lacked. To the best of our knowledge, no scholarly work on PDP research quantified the relationships between scientific literatures from multiple perspectives, as well as revealed how knowledge domains change as society evolves and why these shifts happen, which limits researchers and laypeople to understand the status quo and the emerging trends of this field. To overcome the aforementioned deficiencies, there is an urgent demand to conduct an in-depth bibliometric analysis to bridge the gap between scientific analysis and PDP research.