We have reanalyzed data from both Teasdale et al. (2000)
and Ma and Teasdale (2004)
. These trials recruited participants in eight and four cohorts, respectively, and allocated them to groups (MBCT + TAU vs. TAU alone) within each cohort. However, in the original analyses, cohorts were ignored; the analyses used simple comparisons, based on chi-square tests, odds ratios (ORs), and Cox regression, for relapse and t
tests for the BDI. We have responded to Baldwin et al. (2005)
by including cohort and group effects. We have combined findings for outcomes that were identical in both papers, after checking for homogeneity and goodness of fit across trials. In addition, we have improved the precision of comparisons by adjusting measured outcomes for baseline BDI.
In , we have allowed for cohort by pooling separate ORs for each cohort and by testing whether ORs varied between cohorts. This approach made little difference to the estimated OR, the associated confidence interval (CI), or the significance of the result. Furthermore, the homogeneity test showed that the OR was consistent between cohorts. In other words, MBCT had essentially the same effect in each cohort. The original articles also analyzed survival (“time to relapse”) by Cox regression. We refined this analysis by including both a cohort term and the initial BDI in the model. Although initial BDI contributed significantly to all three Cox regression models and strengthened the effect of MBCT, there was no significant cohort effect. Indeed, cohort contributed less than was expected by chance, in the equivalent of a negative IGC. The analyses in were possible because each MBCT group was selected at random from a given cohort, with the unselected control participants within that cohort forming a matched group. Thus, the effect of MBCT consistently remained significant in all reanalyses and achieved the highest level of significance and the narrowest CIs in the combined analysis.
Relapse at 12 Months (Binary Outcome and Cox Regressions) by Trial
Although each cohort comprises an MBCT group and a control group, we have estimated separate IGCs for each type of group (see ). Homogeneity within control groups is likely to be demographic in origin (Cause A), but MBCT groups may be more homogeneous if groups differed in their effectiveness (Cause B) or if participants tended to follow the responses of others (Cause C). In both studies and the combined analysis, however, all IGCs for time to relapse were negative, which makes all three causes unlikely. Although most but not all IGCs for BDI were positive but generally small, there was no consistent tendency for IGCs to be larger in either MBCT groups or control groups. Furthermore, no IGC was significantly greater or less than zero. Thus, the data in show no evidence of any intragroup dependency.
Time to Relapse and Beck Depression Inventory After Treatment: Comparison Between Randomized Groups
Against this reassuring background, shows the effect of MBCT on BDI after treatment (adjusted for baseline BDI, without and with consideration of cohort effects). These analyses remained significant but showed increased significance levels and wider confidence intervals, as we expected from small positive estimates of IGC. Nevertheless, the combined analyses are still highly significant and confirmed that MBCT improves BDI by 4.7 points (95% CI, 2.1–7.3). also includes the reanalysis of time to relapse, which showed that MBCT increases the average time to relapse by at least 18 weeks (95% CI, 10–27 weeks).
Time to Relapse and Beck Depression Inventory After Treatment: Multilevel Model With Analysis of Covariance to Adjust for Baseline BDI