There are a number of methodological difficulties in constructing a strict placebo intervention that has no clinical effect. The control intervention must be sufficiently convincing in order to motivate participants to maintain compliance but must not itself have a significant specific clinical effect.
Yet, participants who do not experience clinical improvement will tend to drop out of the study or begin to suspect that they have been allocated to the placebo group, thus confounding the significance of the results. In this a comparison group was employed that was likely to elicit some clinical effect, most likely non-specific in nature. This would both ensure subject compliance as well as help differentiate between non-specific effects and any specific effects that may be associated with the SYM method.
Since the SYM method focuses primarily on the experience of mental silence any significant differences in outcome between the two groups could be reasonably attributed to the mental silence construct.