zations have proposed hierarchies of evidence, based on the relative reliability of various types of study designs. [1][2][3][4] Although many people recognize that expert opinions and nonsystematic reviews provide the least reliable level of information, 5,6 such articles continue to have a massive influential presence. 7 Controlled studies assume higher places in hierarchies of evidence than uncontrolled studies, and randomized trials are considered the gold standard for clinical research. [1][2][3][4] However, randomized trials cannot be conducted for all questions of interest 8 and there is debate on whether they give different results than nonrandomized studies. 9-14 Finally, meta-analyses are becoming increasingly frequent in the literature. Meta-analyses are often placed at the highest level of evidence, 1-4 despite their critics. 15,16 No hierarchy of evidence is unanimously accepted.
An important issue is whether the impact of various studies is different and changing over time. Impact on clinical practice and decision making is difficult to measure comprehensively. However, one important measure of impact is the use of citations in the published literature. Citations have limitations, 17 but they provide an objective measurement of how often scientists use a specific published work. One may ask: What is the relative citation impact of published articles using various types of designs? Is this impact commensurate with the proposed hi-erarchies of evidence? Has it changed over time? We aimed to answer these questions using citation analysis.
METHODS
Identification and Eligibility of Relevant Studies
We compared the citation impact across various study designs and between studies published in 1991 and 2001. We searched the Institute for Scientific Information (ISI) Science Citation Index at the Web of Science Database (www.isinet.com) for meta-analyses, randomized controlled trials (RCTs), cohort studies, case-control studies, case reports, nonsystematic reviews, and decision analysis or costeffectiveness analysis records published in 1991 and 2001. These types of publications cover the major, readily identifiable designs used in collecting