The methodological quality of existing research on the benefits of meditation practices is poor because of the varying theoretical approaches and the frequent confirmation bias of individual studies. A 2012 meta-analysis published in "Psychological Bulletin", which reviewed 163 individual studies, tentatively found that Transcendental Meditation produced superior results in "reducing negative emotions, trait anxiety, and neuroticism" as well as improving markers of learning, memory, and self-actualization by comparison with other meditation approaches; the researchers nonetheless recommended improved methodologies for future research. A 2015 systematic review and meta-analysis of 12 studies found that TM may effectively reduce blood pressure compared to control groups, although the underlying studies may have been biased and further studies with better designs are needed to confirm these results. It is generally believed that the quality of research is improving, with some researchers stating that growing scientific evidence supports the therapeutic value of the technique.