The Essence of Meta-analysis: Is It Just a Simple Statistical Evaluation of a Treatment Effect?

We thank Dr Guedes and colleagues’ for their interest and comments on our article. We agree that our study does not include the statistics of a usual metaanalysis. By far the most common use of meta-analysis is ‘‘to summarize anticipated similar effect size estimates in relatively homogenous studies, which are likely to be addressing the same question in similar populations. ’’ This ‘‘analytic meta-analysis’’ approach uses statistics to clarify the overall strength of the particular treatment effect. However, meta-analysis is not limited to this particular approach. In undertaking our study, we certainly did not aim at finding a simple and definitive ‘‘yes or no’’ answer regarding the efficacy of nonpenetrating glaucoma surgery (NPGS), and did not just try to add up the available data using statistics. Our major aim was to evaluate the efficacy of NPGS with regard to glaucoma severity and an appropriately set target intraocular pressure (IOP). We think that a simple and definitive ‘‘yes or no’’ answer regarding the efficacy of any glaucoma treatment—whether it is found to be efficacious or not—would be an oversimplified approach to a complex disease. In meta-analyses, the researchers are constrained by the availability of studies and study results. Among the studies we reviewed—including randomized trials, prospective and retrospective case series—very few studies completely provided data related to glaucoma severity and relevant target IOP. Furthermore, we confronted great heterogeneity in various aspects of the studies: the technique, use of implants/ antimetabolites, postoperative manipulations, available results, etc. When the studies being evaluated are varied and heterogeneous, the use of statistics for estimating an overall treatment effect can be unreliable. Consequently, for our study the regular use of statistics was not justifiable. When the studies being evaluated are varied, heterogeneous or grouping is difficult, an ‘‘exploratory or causal meta-analysis’’ approach can focus ‘‘on explaining why the effect sizes vary or are dissimilar in heterogeneous studies addressing the same question,’’ and can help raise issues to be addressed in future studies. In our study, we tried to question the possible causes of the great variability in the results of NPGS, hence provided data related to goniopuncture rates, postoperative 5-flourouracil application rates, use of implants/antimetabolites, and follow-up durations when available. We appreciate the work of Cheng et al related to efficacy of NPGS. However, we think that evaluation of success of any glaucoma therapy with a mere 21mm Hg target is no longer valid and can be misleading. We emphasized this with the statement ‘‘without data relevant to glaucoma severity, it is not possible to properly evaluate the efficacy and success of any glaucoma therapy’’ and suggested that ‘‘studies with data related to glaucoma severity and proper target IOPs are required. ’’ We also agree with Dr Guedes and colleagues’ that ideally a meta-analysis should include studies published in languages other than English. However, English continues to dominate the research communications as the ‘‘‘lingua franca,’’ and the