Fig. 4From: Changing trends in clinical research literature on PubMed database from 1991 to 2020Trend chart of percentage change of each research type in clinical research literature in PubMed database from 1991 to 2020. A Trend change of primary literature proportion; B trend change of secondary literature proportion. RCT: randomized clinical trial; PCT: pragmatic clinical trial; Sys-meta: systematic review and meta-analysisBack to article page