I’ve typically mocked the use of effect sizes in research, but perhaps I need to be a bit more open minded. This paper looked at the “minimally important difference” (note: not quite the same thing as the minimal clinically important difference) across 33 published studies of health related quality of life measures. Even though the structure of many of these measures was radically different, the minimally important difference was almost always close to 0.5. The authors draw an analogy to measurement on a seven point scale, where one unit is understood from previous psychological research to represent (roughly) the limit of human discrimination.

Norman GR, Sloan JA, Wyrwich KW. Point/Counterpoint. Interpretation of Changes in Health-related Quality of Life: The Remarkable Universality of Half a Standard Deviation. Medical Care: May 2003, 41(5): 582-592.

You can find an earlier version of this page on my blog.