Web of Science Core Collection's coverage expansion:The forgotten Arts & Humanities Citation Index?

The expansion of Web of Science Core Collection (WoSCC) over the recent years has partially accounted for the "norm" of growth of research output in many bibliometric analysis studies. However, the expansion patterns of different citation indexes may be different, which may benefit some disciplines but hinder others. Utilizing Science Citation Index Expanded (SCIE), Social Sciences Citation Index (SSCI), and Arts & Humanities Citation Index (A&HCI), this study attempts to elaborate on WoSCC's coverage expansion patterns among these three databases from 2001 to 2020. Results show that different from SCIE/SSCI, both the annual publication volumes in the A&HCI database and all A&HCI journals have remained relatively stagnant in all document types considered scenario or have gained relatively slight increases in only citable items considered scenario. Although the number of A&HCI journals also has increased remarkably, the average journal publication volume of A&HCI journals has decreased gradually if all document types are considered or kept relatively stagnant when citable items only are considered. Besides, the A&HCI database has ceased the systematic index of individually selected items from SCIE/SSCI journals since 2018. The study finally discusses the possible causes and consequences of the unbalanced expansion of WoSCC's different citation indexes.