Background: Level of evidence (LoE) is a hierarchical system for classifying the quality of studies.
Objectives: This study examined the factors affecting the number of citations to clinical articles related to the treatment of human diseases that have included the LoE in their abstracts.
Methods: A total of 3,683 therapeutic articles published between 2011 and 2013 that mentioned the LoE in their abstract and were indexed in PubMed and Web of Science were retrieved. The LoE and type of study design were extracted from abstracts and other bibliographic and citation information was obtained from PubMed and Web of Science databases. Independent samples t-test, one-way ANOVA, Pearson correlation test and linear regression were used to analyze the relationship between the variables.
Results: Articles with level I evidence had the lowest frequency (290, 7.9%) and articles with level IV had the highest frequency (1,831, 49.7%). Five-year citations ranged from zero to 215, with a median of 13 citations. The median values of five-year citations from level I to level V were 20.5, 15, 14, 11, and 6 citations, respectively. Evaluation of the models to examine the factors affecting the number of citations showed that the change of evidence-level from level I to V reduced the number of citations (P < 0.001).
Conclusions: Journal Impact Factor, LoE, number of references, number of authors, number of title words, number of pages, article type and subject category accounted for about 25% of the variation in five-year citations of clinical papers. Clinical papers with high LoE (levels I & II) received more citations over a five-year period than those with lower LoE (levels III & IV).