A 2007 review of 13 studies found evidence for the efficacy of hypnosis in the reduction of pain in some conditions. However the studies had some limitations like small study sizes, bringing up issues of power to detect group differences, and lacking credible controls for placebo or expectation. The authors concluded that "although the findings provide support for the general applicability of hypnosis in the treatment of chronic pain, considerably more research will be needed to fully determine the effects of hypnosis for different chronic-pain conditions."