Meta-Analytic Methods
Data extraction was performed by two independently working reviewers (MHB and JH) on
specially designed Microsoft Excel spreadsheets. Reviewers collected data on methods,
participants, intervention and outcome measurements, and other relevant attributes and
results of the studies. Any disagreement between reviewers was resolved through discussion and obtaining more information from the study investigators when possible.
The outcome measure selected from each included trial was the difference in mean
improvement between the omega-3 fatty acid and placebo group in a clinical rating scale
measuring depression severity over the course of the trial. Preferred rating scales for
measuring depression severity were the Hamilton Depression Rating Scale (HDRS), either
the 9-item short form, 17-item, 21-item or 25-items scales, and the Montgomery Åsberg
Depression Rating Scale (MADRS). When available, HDRS score from each study was used. If the HDRS was not available we used the MADRS. If neither HDRS nor MADRS data were available, we used the clinician rated measure of depression that the investigators
identified as their primary outcome. When no clinician-rated measures of depression
severity were available from a trial then self-report measures were utilized. When the
standard deviation of the mean improvement on placebo or omega-3 fatty acids was not
reported in individual studies this was imputed based on the standard deviation of reported baseline and endpoint depression severity using Cochrane methodology.16 The formula SD(improvement) = √(SD(baseline)2+SD(endpoint)2-2*r*SD(baseline)*SD(endpoint) where r=correlation coefficient was used to calculate the standard deviation of mean improvement. A correlation coefficient of 0.4 was used for this calculation based on the average of computed correlation coefficients in included studies where all information on standard deviations was available.