Is success in weight loss treatment contagious (do attendance and outcomes cluster within treatment groups)?

Data from a trial of a 26-session structured group behavioral weight loss intervention were used to examine how group attendance, weight loss, and changes in depression clustered within therapy groups. Participants were recruited via a population-based survey of female health plan members aged 40-65. The sample included 143 women attending 13 therapy groups. Average number of sessions attended was 15.9 (S.D. 7.2) and average weight loss over 12 months was 4.24 kg (S.D. 7.79 kg). In hierarchical (random effects) linear models predicting attendance and weight loss, attendance did cluster significantly within therapy groups (F = 2.83, df = 12, p = .002; ICC = .14) but weight loss (F = 0.89, df = 12, p = .56; ICC = .00) and change in depressive symptoms (F = 0.25, df = 12, p = .99; ICC = .00) did not. Accounting for baseline characteristics of group participants had no significant effect on these findings. Relatively small average weight loss in this sample may have limited our ability to detect clustering of weight loss within groups. We conclude that clinicians should consider addressing the effects of drop-out on other group members and that researchers should consider the impact of clustering in analyses of data regarding group treatments.: