Evaluating heterogeneity in cumulative meta-analyses

Cumulative meta-analyses of five smoking cessation interventions (clonidine, nicotine replacement therapy using gum and patch, physician advice and acupuncture) were used to calculate I2 and H. These values were plotted by year of publication, control event rate and sample size to trace the development of heterogeneity over these covariates.The cumulative evaluation of heterogeneity varied according to the measure of heterogeneity used and the basis of cumulation. Plots produced from the calculations revealed areas of heterogeneity useful in the consideration of potential sources for further study.The examination of heterogeneity in conjunction with summary effect estimates in a cumulative meta-analysis offered valuable insight into the evolution of variation. Such information is not available in the context of conventional meta-analysis and has the potential to lead to the development of a richer picture of the effectiveness of interventions.As predicted by Mulrow [1,2] (among others), reports of meta-analyses – the suite of statistical techniques used to arrive at pooled estimates of effects across a series of studies (often but not always) during the course of a systematic review – have ballooned in parallel with the rapid and sustained pace at which information becomes available about the efficacy of interventions. Often, meta-analyses are conducted after a collection of studies have been identified; statistical pooling occurs at one point in time. The sequential pooling of the effect estimate in a "cumulative" manner as studies are published or according to other specific variables of interest (study quality or control event rate, for instance) were described and developed by Lau and colleagues [3-5]. These developments have focused on describing the evolution of the point estimate and its confidence intervals.Conventional meta-analyses are usually reported in conjunction with a test for heterogeneity. A popular statistic, Cochran's Q, is the sum of the squared