Our main aim was to examine whether the expected beneficial effects of a mindfulness-based stress-reduction intervention (MBSR) (1) are attributable to actual changes in mindfulness skills.
First, it had to be shown that MBSR had the hypothesized effects in our sample of distressed individuals from the general population. We did replicate previous findings regarding the effectiveness of MBSR on decrease of feelings of distress [9
] and increase of quality of life [11
], and extended previous results to other measures of distress (vital exhaustion), mood, and quality of life than those applied earlier. The general convergence of findings underscores the fact that previous results are not limited to the specific measures used. Some nuances should be acknowledged, however. In our study, quality of life seems less sensitive to change than measures of psychological distress and positive affect. This is in line with research on temporal changes in quality of life in patient populations, indicating fairly stable levels [41
]. In addition, decrease in negative affect showed no difference between the groups, probably because the control group decreased significantly as well. This may have been due either to natural improvement in mood over the course of time, to the expectancy of the participants of coming improvement as the intervention would start soon, or even just to the statistical effect of regression to the mean.
Regarding the main aim of our investigation, support was obtained for at least a partial mediation effect by mindfulness. This is the first study to show such an effect in a controlled study. Only few previous investigations have examined a mediation effect by mindfulness skills. While several studies reported correlations between change in mindfulness skills and decrease in feelings of distress [20
], a mediation effect of mindfulness could not be obtained due to the lack of a control group. The only study on the mediation issue that did apply a randomized controlled design did not find a correlation between change in mindfulness skills and change in outcome variables [42
]. However, this was probably due to the fact that their sample consisted of students without any psychological symptoms. Perhaps because of our use of a truly distressed sample in a randomized study, we have been able to demonstrate an at least partial mediation effect of mindfulness in the case of some outcome variables.
We found evidence for a mediation effect of general mindfulness in the case of perceived stress, quality of life, and partially on vital exhaustion. For the facet accepting without judgment and observing mainly sensory phenomena, no mediation effects were found, although change in accepting without judgment did correlate with improvement in quality of life over time. A potential explanation of these findings may be that the focus of the intervention is on enhancing mindfulness in general, which is more strongly tapped by the MAAS, than on the specific aspects of accepting ones feelings and thoughts, and observing bodily and physical sensations, although these aspect are also addressed during the intervention.
Regarding relationships between outcome and home practice or attendance of sessions, we found no effects. The results concerning attendance are in line with three out of five studies that have examined this effect and found none [11
]. Two other studies did report a positive effect of attendance on outcome [10
]. Ceiling or floor effects cannot explain this inconsistency as the number of attended sessions ranged from relatively low [43
] to high [11
]. Also the power to detect effects did not differ systematically between the studies. One may speculate that practice is more important for achieving positive effects than session attendance, which however, we did not observe either.
Previous studies on mindfulness-related interventions have reported mixed results regarding the relation between the amount of home practice and outcome. Clear consistent associations have been obtained in some studies [10
], while in most studies, no or inconsistent associations were found [9
]. Some but not all null findings may have been due to either relatively low initial levels of distress [11
] or ceiling effects of the amount of practice [11
]. However, studies reporting significant associations were based on larger samples (61
121 in the treatment group) than studies finding no or inconsistent associations (14
59), suggesting a power problem in many studies, including ours, to detect an effect of probably modest magnitude. Although it cannot be ruled out, a more fine-grained assessment of the amount of minutes spent in practice may not have produced stronger associations in our study, as most studies finding no or inconsistent associations did assess the number of minutes practicing.
One limitation of the present investigation is the relatively small sample size, which prevents the detection of small effects. Importantly, our results are restricted to immediate effects post-intervention. The investigation of potentially mediating effects of mindfulness skills on long-term outcome is very much needed. Furthermore, although we attempted for neutral wording in our advertisements, emphasizing the stress reduction aspect, our recruitment procedure by means of advertisements may have partially attracted a subset of people interested in this form of intervention, limiting the generalizability to the general population.