Longitudinal analysis of the quantity and classification of clinical research literature in the PubMed database between 1991 and 2020
Authors: Zhao Xiyi , Jiang Hao , Yin Jianyun , Liu Hongchao , Zhu Ruifang , Mei Shencong , Zhu Changtai 
Published: March 2022

Abstract
Background: Clinical research publications have become the dominant source and basis of clinical evidence-based decision-making, and the PubMed database has the most extensive application foundation in the world. Exploring the type and quantity of clinical research publications in the PubMed database is useful for clarifying the changing trends of clinical research development in recent years. Therefore, a longitudinal analysis of the type and quantity of clinical research publications in the PubMed database over three decades was conducted and combined with evidence-based medicine to provide reference for the rational allocation of clinical research resources in future.Methods: The PubMed database was searched to retrieve clinical research according to the type and year of publication from January 1, 1991 to December 31, 2020. The research types were classified as primary and secondary literature using randomized controlled trial (RCT), non-randomized controlled trial (non-RCT), and pragmatic clinical trial (PCT); cohort, case control, and cross-sectional study; case report/series; guideline; expert consensus; narrative, systematic, umbrella, and scoping review; traditional meta-analysis (Tra-meta), systematic review and meta-analysis (Sys-meta), and network meta-analysis (Net-meta). The statistical analyses focused on identifying the changes in the quantity, annual growth rate, and 30-year average growth rate of each specific study type in primary and secondary literature between 1991 and 2020; changes in the constituent proportion of each specific study type in primary and secondary literature; and changes in the ratio of the quantity of guidelines and meta-analysis to primary literature.Results: A total of 1,078,404 primary literatures were retrieved and the constituent proportions were ranked from high to low as case report/series (27.54%), RCT (23.62%), cohort study (21.05%), cross-sectional study (17.49%), case control study (9.15%), non-RCT (1.01%), and PCT (0.15%). Correspondingly, 1,302,173 secondary literatures were retrieved and ranked as narrative review (70.88%), systematic review (15.02%), Sys-meta (13.89%), Tra-meta (4.48%), expert consensus (2.31%), guidelines (1.49%), scoping review (0.68%), Net-meta (0.40%), and umbrella review (0.04%). The average annual growth rate for the primary literature was 10.28%, and ranked from high to low as PCT (83.68%), cohort study (17.74%), cross-sectional study (17.61%), non-RCT (12.11%), case control study (8.86%), RCT (7.68%), case report/series (7.51%); while that for the secondary literature was 10.57%, and ranked from high to low as Net-meta (48.97%), umbrella review (47.09%), scoping review (41.92%), Sys-meta (33.44%), systematic review (33.05%), Tra-meta (12.49%), expert consensus (9.22%), narrative review (8.72%), and guidelines (2.82%). The analysis based on the ratio of the number of secondary literature to the primary literature, the guidelines to primary literature ratio decreased year by year, from 12.08% in 1991 to 1.39% in 2020, for a total ratio of 3.28%, while the meta-analysis to primary literature ratio increased year by year, from 4.59% in 1991 to 27.76% in 2020, for a total ratio of 21.04%.Conclusion: Both the structure and number of clinical studies changed significantly from 1991 to 2020. Based on the trend, the case report/series, case control study, and narrative review are on the decline, while cohort study, cross-sectional study, systematic review, and Sys-meta have increased. Combined with the grade standard of evidence-based medicine, we recommend enhancing the focus on RCT and cohort study when allocating research resources, as well as improving the quality rather than quantity of Sys-meta and systematic reviews in future.