A systematic review of 400 clinical trials on meditation (published 1956-2005) found that most studies have poor methodological quality, with significant threats to validity across all major quality domains. The five broad categories of meditation practices studied were mantra meditation, mindfulness meditation, yoga, t'ai chi, and qigong. The most common clinical conditions examined were hypertension, miscellaneous cardiovascular diseases, and substance abuse. Although methodological quality has improved over time by 0.014 points per year, future trials must be more rigorous in design, execution, analysis, and reporting.
A panel of 7 meditation experts reached consensus through a 5-round Delphi study on essential criteria for defining meditation for systematic review. Essential features include a defined technique, logic relaxation, and a self-induced state. Optional features include psychophysical relaxation, a self-focus skill or anchor, altered consciousness, mystical experience, suspension of logical thought, embedding in a religious/spiritual/philosophical context, or mental silence. The results highlight challenges in distinguishing meditative from nonmeditative practices and offer an approach to operationalizing meditation for comparative research.