There are few high quality studies such as randomized controlled trials looking at the efficacy of treating asthma with "breathing retraining" methods in general, which include the Buteyko method, yoga training and other relaxation techniques. Many of the studies that have evaluated breathing retraining have significant methodological flaws, including small sample sizes, possible patient selection bias as well as heterogeneity in design that makes coming to a firm conclusion difficult. These studies are also hampered by the difficulty in proper blinding and placebo control which could introduce more bias into these studies.