Bias and small-study effects influence treatment effect estimates: a meta-epidemiological study in oral medicine.

OBJECTIVES: To examine the influence of the following study characteristics on their study effect estimates: (1) indexing in MEDLINE, (2) language, and (3) design. For randomized trials, (4) trial size and (5) unequal randomization were also assessed. STUDY DESIGN AND SETTING: The CAtegorical Dental and Maxillofacial Outcome Syntheses meta-epidemiologic study was conducted. Eight databases/registers were searched up to September 2012 for meta-analyses of binary outcomes with at least five studies in the field of dental and maxillofacial medicine. The previously mentioned five study characteristics were investigated. The ratio of odds ratios (ROR) according to each characteristic was calculated with random-effects meta-regression and then pooled across meta-analyses. RESULTS: A total of 281 meta-analyses were identified and used to assess the influence of the following factors: non-MEDLINE indexing vs. MEDLINE indexing (n = 78; ROR, 1.12; 95% confidence interval [CI]: 1.05, 1.19; P = 0.001), language (n = 61; P = 0.546), design (n = 24; P = 0.576), small trials (<200 patients) vs. large trials (>/=200 patients) (n = 80; ROR, 0.92; 95% CI: 0.87, 0.98; P = 0.009) and unequal randomization (n = 36; P = 0.828). CONCLUSION: Studies indexed in MEDLINE might present greater effects than non-indexed ones. Small randomized trials might present greater effects than large ones.