Fig. 2From: Changing trends in clinical research literature on PubMed database from 1991 to 2020Quantity change trend of each study type in clinical research literature on PubMed database from 1991 to 2020. A Trends in the number of primary literature; B trends in the number of secondary literature. RCT randomized clinical trial; PCT pragmatic clinical trial; Sys-meta systematic review and meta-analysisBack to article page