Type D caseness
When comparing pre-to post-treatment changes in Type D caseness between the two groups, 9 (to 10 in one imputation) participants out of 41 (22–24 %) Type D individuals at baseline in the intervention group changed to non-Type D post-intervention, compared to 7 out of 43 (16 %) in the control group. Among non-Type D individuals at baseline, 2–4 out of 31 (7–13 %) in the intervention group changed to Type D post-intervention, which was 2–3 out of 31 (7–10 %) in the control group. These changes were not different between groups (χ
2
(2) < 1.59, p > 0.45).
The role of mindfulness and practice
In an omnibus test of change from pre-to post-intervention of all three mindfulness subscales, a significant main effect of time (intercept: pooled t(142) = 4.03, p < .001, partial η
2
= 0.26) and a significant effect of group (pooled t(142) = 4.25, p < .001, partial η
2
= 0.28) emerged. In univariate analyses, the difference between groups was significant for all three mindfulness subscales; for general mindfulness (pooled t(144) = 4.57, p < .001, partial η
2
= 0.13), Accept Without Judgment (pooled t(144) = 2.82, p = .005, partial η
2
= 0.06), and Observe (pooled t(144) = 5.44, p < .001, partial η
2
= 0.17), reflecting a larger increase in mindfulness scores in the intervention group compared to the control group.
In the whole sample, increase in general mindfulness was associated with decreases in NA (pooled r = −0.35, p < .001), SI (r = −0.24, p = .004), and state negative affect (r = −0.25, p = .002) from baseline to post-intervention. An increase in the Accept Without Judgment subscale was associated with a decrease in NA (r = −0.31, p < .001), but not with changes in SI or state negative affect (p > .10), while an increase in the Observe subscale was related to a decrease in SI (r = −0.18, p = .03), but not with changes in NA or state negative affect (p > .10).
Thus, conditions were met for a potential mediation effect by general mindfulness regarding both NA and SI, by Accept Without Judgment regarding NA and by Observe regarding SI. A final ANCOVA analysis was conducted to examine the mediation effects on NA. This analysis was similar to the original analysis on effects on NA, except that in addition to change in state negative affect, pre-post intervention change scores in general mindfulness and Accept Without Judgment were added as covariates. In this analysis, the original group effect was strongly reduced (pooled t(142) = 1.97, p = .05, partial η
2
= 0.03). The effects of change in general mindfulness (pooled t(142) = 1.99, p = .047, partial η
2
= 0.03) and change in Accept Without Judgment (pooled t(142) = 2.67, p = .008, partial η
2
= 0.05) were significant. The bootstrap analyses for mediation effects with 5,000 resamples for both mediators in isolation indicated a significant mediation effect by both general mindfulness (coefficient = 0.63, 95 % CI = 0.18–1.22) and Accept Without Judgment (coefficient = 0.44, 95 % Confidence Interval = 0.02–1.00).
In a similar ANCOVA analysis examining the mediation effects on SI, change in general mindfulness and change in Observe were introduced as covariates. The original group effect was reduced to nonsignificance (pooled t(142) = 1.36, p > .10). However, the effect of change in Observe was not significant either (pooled t(142) = 0.97, p > .10), while the effect of general mindfulness approached significance (pooled t(142) = 1.78, p = .075, partial η
2
= 0.02). The bootstrap analyses for mediation effects for both mediators in isolation indicated a significant mediation effect by change in general mindfulness (coefficient = 0.33, 95 % Confidence Interval = 0.02–0.72) but not by change in Observe (coefficient = 0.25, 95 % CI = −0.15–0.74).
The treatment group practiced on average 4.70 (SD = 1.48) times a week during the entire 8-week intervention period. No associations were found between amount of weekly formal home practice and changes in NA, SI or state negative affect. Of associations with baseline values, only an inverse association was found between weekly home practice and baseline NA (r = −0.40, p = .001). Regarding session attendance, no associations were found between the number of sessions a participant attended (mean = 4.54; SD = 1.45) and change in NA, SI or state NA.