Epidemiology and Reporting Characteristics of Systematic Reviews

Background: Systematic reviews (SRs) have become increasingly popular to a wide range of stakeholders. We set out to capture a representative cross-sectional sample of published SRs and examine them in terms of a broad range of epidemiological, descriptive, and reporting characteristics, including emerging aspects not previously examined. Methods and Findings: We searched Medline for SRs indexed during November 2004 and written in English. Citations were screened and those meeting our inclusion criteria were retained. Data were collected using a 51-item data collection form designed to assess the epidemiological and reporting details and the bias-related aspects of the reviews. The data were analyzed descriptively. In total 300 SRs were identified, suggesting a current annual publication rate of about 2,500, involving more than 33,700 separate studies including one-third of a million participants. The majority (272 [90.7%]) of SRs were reported in specialty journals. Most reviews (213 [71.0%]) were categorized as therapeutic, and included a median of 16 studies involving 1,112 participants. Funding sources were not reported in more than one-third (122 [40.7%]) of the reviews. Reviews typically searched a median of three electronic databases and two other sources, although only about two-thirds (208 [69.3%]) of them reported the years searched. Most (197/295 [66.8%]) reviews reported information about quality assessment, while few (68/294 [23.1%]) reported assessing for publication bias. A little over half (161/300 [53.7%]) of the SRs reported combining their results statistically, of which most (147/161 [91.3%]) assessed for consistency across studies. Few (53 [17.7%]) SRs reported being updates of previously completed reviews. No review had a registration number. Only half (150 [50.0%]) of the reviews used the term “systematic review” or “meta-analysis” in the title or abstract. There were large differences between Cochrane reviews and non-Cochrane reviews in the quality of reporting several characteristics. Conclusions: SRs are now produced in large numbers, and our data suggest that the quality of their reporting is inconsistent. This situation might be improved if more widely agreed upon evidence-based reporting guidelines were endorsed and adhered to by authors and journals. These results substantiate the view that readers should not accept SRs uncritically. Data were collected on the epidemiological, descriptive, and reporting characteristics of recent systematic reviews. A descriptive analysis found inconsistencies in the quality of reporting. Background.: In health care it is important to assess all the evidence available about what causes a disease or the best way to prevent, diagnose, or treat it. Decisions should not be made simply on the basis of—for example—the latest or biggest research study, but after a full consideration of the findings from all the research of good quality that has so far been conducted on the issue in question. This approach is known as “evidence-based medicine” (EBM). A report that is based on a search for studies addressing a clearly defined question, a quality assessment of the studies found, and a synthesis of the research findings, is known as a systematic review (SR). Conducting an SR is itself regarded as a research project and the methods involved can be quite complex. In particular, as with other forms of research, it is important to do everything possible to reduce bias. The leading role in developing the SR concept and the methods that should be used has been played by an international network called the Cochrane Collaboration (see “Additional Information” below), which was launched in 1992. However, SRs are now becoming commonplace. Many articles published in journals and elsewhere are described as being systematic reviews. Why Was This Study Done?: Since systematic reviews are claimed to be the best source of evidence, it is important that they should be well conducted and that bias should not have influenced the conclusions drawn in the review. Just because the authors of a paper that discusses evidence on a particular topic claim that they have done their review “systematically,” it does not guarantee that their methods have been sound and that their report is of good quality. However, if they have reported details of their methods, then it can help users of the review decide whether they are looking at a review with conclusions they can rely on. The authors of this PLoS Medicine article wanted to find out how many SRs are now being published, where they are being published, and what questions they are addressing. They also wanted to see how well the methods of SRs are being reported. What Did the Researchers Do and Find?: They picked one month and looked for all the SRs added to the main list of medical literature in that month. They found 300, on a range of topics and in a variety of medical journals. They estimate that about 20% of reviews appearing each year are published by the Cochrane Collaboration. They found many cases in which important aspects of the methods used were not reported. For example, about a third of the SRs did not report how (if at all) the quality of the studies found in the search had been assessed. An important assessment, which analyzes for “publication bias,” was reported as having been done in only about a quarter of the cases. Most of the reporting failures were in the “non-Cochrane” reviews. What Do These Findings Mean?: The authors concluded that the standards of reporting of SRs vary widely and that readers should, therefore, not accept the conclusions of SRs uncritically. To improve this situation, they urge that guidelines be drawn up regarding how SRs are reported. The writers of SRs and also the journals that publish them should follow these guidelines. Additional Information.: Please access these Web sites via the online version of this summary at http://dx.doi.org/10.1371/journal.pmed.0040078.