Authors' Reply to Response to “Mindfulness and Psychological Well-Being: Are They Related to Type of Meditation Technique Practiced?”

Dear Editor:

Dr. Travis has raised a number of important points in his letter, especially the one regarding potential confounding effects of covariates. ¹ As Dr. Travis points out, this issue involves a classic problem in research that is often neglected.

In line with the classic article he refers to, ² he makes a distinction between two classes of variables that were used as covariates in our study: (1) demographic variables, and (2) process variables, such as the amount and intensity of meditation practice. Both classes of variables differed between our Transcendental Meditation (TM) and Mindfulness Meditation (MM) groups, and were, therefore controlled in the analyses. Theoretically, differences in means between the groups may have been overcorrected in the case where covariates were correlated with treatment effect. We have reanalyzed the data more closely to examine this.

In completely unadjusted analyses, the TM group would show significantly higher scores on general mindfulness, psychologic quality of life (QoL-P), and positive affect, and lower scores on perceived stress compared with the MM group. However, because the TM group was significantly older and contained more men, we controlled for these demographic characteristics, which abolished the differences found in unadjusted analyses. The problem of bias Dr. Travis is referring to may have occurred in instances where the covariates correlated with treatment outcome. In our case, this applies to QoL-P and perceived stress (older and male meditators appeared to significantly perceive less stress, and to experience a higher psychologic quality of life), but not to general mindfulness and positive affect. The findings regarding perceived stress and QoL-P are therefore difficult to interpret. In the uncorrected analyses, the differences between groups are likely to be biased by the unequal demographics among groups (and possibly by even more variables that were not measured), while in the corrected analyses, the possibility of overcorrection indeed cannot be ruled out, as Dr. Travis suggests.

Regarding process variables, the number of days per week meditating was the only variable that was associated with various indices of both mindfulness and psychologic well-being. ³ When included in the analyses on the association between type of meditation and indices of mindfulness and psychologic well-being, even without the inclusion of age and sex, the differences between groups found in unadjusted analyses were no longer significant. The number of days of meditation per week also correlated with all outcome variables that showed differences between the meditation groups. These findings together may also be interpreted in terms of a mediation effect, with differences between the groups on outcome variables being a consequence of simply more frequent meditations per week in the TM group.

In general, we agree that the differences in psychologic well-being and mindfulness between the groups are difficult to interpret in light of the large demographic and process differences. Nevertheless, the present study has shown the potential importance of the frequency of meditation, while the importance of the kind of meditation practice is less clear. However, a definite answer can only be provided by conducting a randomized trial with more experimental control regarding meditation frequency and duration while comparing TM and MM over the course of time in previously meditation-naïve participants.

Regarding previous studies mentioned by Dr. Travis, we would like to state that his review of the literature is highly selective, incorrectly suggesting that TM is more effective in decreasing psychologic symptoms compared to MM. The cited meta-analysis on effectiveness of mindfulness-based interventions in medical patient groups by Bohlmeijer and colleagues ⁴ has been criticized (containing too few studies and selectively using outcome variables), ⁵ while a much larger meta-analysis has shown medium-to-large effect sizes of mindfulness-based interventions on symptoms of anxiety and depression in mental health patient groups. ⁶

We agree with Dr. Travis that more research is needed, and collaboration between researchers from different meditation traditions would be very helpful. However, instead of focusing on existing meditating populations, as suggested by Dr. Travis, a focus on randomized controlled trials involving participants previously naïve regarding meditation and including long follow-up measurements is crucial. We are willing to take up this challenge and are looking forward to future collaborative studies.