Relative citation impact of various study designs in the health sciences

Context The relative merits of various study designs and their placement in hierarchies of evidence are often discussed. However, there is limited knowledge about the relative citation impact of articles using various study designs. Objective To determine whether the type of study design affects the rate of citation in subsequent articles. Design and Setting We measured the citation impact of articles using various study designs-including meta-analyses, randomized controlled trials, cohort studies, case-control studies, case reports, nonsystematic reviews, and decision analysis or cost-effectiveness analysis-published in 1991 and in 2001 for a sample of 2646 articles. Main Outcome Measure The citation count through the end of the second year after the year of publication and the total received citations. Results Meta-analyses received more citations than any other study design both in 1991 (P<.05 for all comparisons) and in 2001 (P<.001 for all comparisons) and both in the first 2 years and in the longer term. More than 10 citations in the first 2 years were received by 32.4% of meta-analyses published in 1991 and 43.6% of meta-analyses published in 2001. Randomized controlled trials did not differ significantly from epidemiological studies and nonsystematic review articles in 1991 but clearly became the second-cited study design in 2001. Epidemiological studies, nonsystematic review articles, and decision and cost-effectiveness analyses had relatively similar impact; case reports received negligible citations. Meta-analyses were cited significantly more often than all other designs after adjusting for year of publication, high journal impact factor, and country of origin. When limited to studies addressing treatment effects, meta-analyses received more citations than randomized trials. Conclusion Overall, the citation impact of various study designs is commensurate with most proposed hierarchies of evidence.