The Efficacy and Effectiveness of Online CBT - Jeroen Ruwaard

5.3. Results 93include data of the waiting-list, because the participants in this group provided followupdata at a later time. Binge eating and purging frequency data were highly skewed.Consequently, we reverted to rank-score ANCOVA (Conover & Iman, 1982; LaVange& Koch, 2006) to assess group differences with these data, and chose to summarizethese variables as medians and interquartile ranges. Analyses included all participants,irrespective of treatment adherence and attrition. Missing values were handledthrough last-observation-carried-forward (LOCF) data imputation. Within-group gainscores and between-group differences were standardized to Cohen’s d, using thepooled standard deviation of the pretest scores as the standardizer (J. Cohen, 1988).Confidence intervals around d were approximated from the central t-distribution(Robey, 2004).Clinical significance. We tested the differential probability of a clinically relevantoutcome after online treatment compared to both control groups with two-sidedFisher’s exact tests, and expressed this difference as odds ratios (OR; Hillis & Woolson,2002). For binge eating and purging, we defined abstinence (i.e., a frequency of 0)as the clinically relevant outcome. With regard to EDE-Q and BAT, we applied theprinciples of reliable clinically significant change (RCSC; Jacobson & Truax, 1991).We used the reliable change index to determine how many scale points a participanthad to change to rule out measurement error (EDE-Q: 1.1 points; BAT: 18.5 points).RCSC was defined as reliable change from a pretest score above the clinical cut-off(EDE-Q: < 2.3; BAT: < 36) to a posttest score below the cut-off .5.3 Results5.3.1 ParticipantsRecruitment resulted in 273 respondents. Of these, 91 (33%) did not complete thescreening. Of the remaining 182 respondents, 105 (58%) met inclusion criteria (cf.Figure 5.1). As shown in Table 5.1, baseline characteristics were equally distributedacross the three experimental groups. Participants were female (n = 104; 99%)and about 31 years old. Self-reported illness history indicated chronic symptoms(the average duration of symptoms was M = 11 years, SD = 9), for which 62%