Effect size of:
.2 Small effect
.5 Medium effect
.8 Large effect
Cohen, J. Statistical power for the behavioral sciences (2nd ed.). Hillsdale, NJ: Erlbaum (1988).
Page 5 of http://www.wmich.edu/evalphd/wp-content/uploads/2010/05/Effect_Size_Substantive_Interpretation_Guidelines.pdf
"Cohen’s benchmarks Cohen (1988) attempted to address the issue of interpreting effect size estimates relative to other effect sizes. He suggested some general definitions for small, medium, and large effect sizes in the social sciences. However, Cohen chose these quantities to reflect the typical effect sizes encountered in the behavioral sciences as a whole -- he warned against using his labels to interpret relationship magnitudes within particular social science disciplines or topic areas. His general labels, however, illustrate how to go about interpreting relative effects. Cohen labeled an effect size small if d = .20 or r = .10. He wrote, "Many effects sought in personality, social, and clinical-psychological research are likely to be small . . . because of the attenuation in validity of the measures employed and the subtlety of the issue frequently involved" (p. 13). Large effects, according to Cohen, are frequently "at issue in such fields as sociology, economics, and experimental and physiological psychology, fields characterized by the study of potent variables or the presence of good experimental control or both" (p. 13). Cohen suggested large magnitudes of effect were d = .80 or r = .50. Medium-sized effects were placed between these two extremes, that is d = .50 or r = .30. A caution against using Cohen’s benchmarks as generic descriptors of the magnitude of effect size is implied above. Because some areas, like education, are likely to have smaller effect sizes than others, using Cohen’s labels may be misleading."