It seems logical that experienced meditators would be more likely to be able to generate the experiential and physiological changes associated with meditation at a magnitude sufficient for detection; however selecting them from the wider population necessarily precludes the use of randomisation. This weakens the likelihood that the intervention and comparison groups are truly homogenous. In order to compensate for this comparison participants can be selected to match key parameters. In the case of meditation research, these parameters should include an interest in and motivation to practice meditation.
Ramesh Manocha