A test for funnel plot asymmetry, described by Egger et al. (Egger et al. 1997), was applied to assess evidence for publication bias. The test used a linear regression approach to measure funnel plot asymmetry on the natural logarithm of the OR. Degree of funnel plot asymmetry was measured by the intercept from regression of standard normal deviates against precision. The larger the deviation of each study from the funnel curve, the more pronounced the asymmetry. Results from small studies are expected to scatter widely at the bottom of the graph, with the spread narrowing among larger studies. The significance of the intercept was evaluated using the T test. The classic fail-safe analysis (Rosenthal 1979) was also applied to address potential publication bias. If publication bias is found, the “Duval and Tweedie's Trim and Fill” procedure (Duval and Tweedie 2000) is used to impute the number of potentially-missing studies. The Trim and Fill procedure imputes these missing studies, adds them to the analysis, and then re-computes the adjusted overall effect size.
