Rules of thumb for interpreting effect sizes

Cohen's Standard	Effect Size	Percentile Standing	Percent of Nonoverlap
	2.0	97.7	81.1%
	1.9	97.1	79.4%
	1.8	96.4	77.4%
	1.7	95.5	75.4%
	1.6	94.5	73.1%
	1.5	93.3	70.7%
	1.4	91.9	68.1%
	1.3	90	65.3%
	1.2	88	62.2%
	1.1	86	58.9%
	1.0	84	55.4%
	0.9	82	51.6%
LARGE	0.8	79	47.4%
	0.7	76	43.0%
	0.6	73	38.2%
MEDIUM	0.5	69	33.0%
	0.4	66	27.4%
	0.3	62	21.3%
SMALL	0.2	58	14.7%
	0.1	54	7.7%
	0.0	50	0%

contributed by Frank LaBanca, EdD

Interpretation of Cohen's d

Effect sizes can be thought of as the average percentile standing of the average treated (or experimental) participant relative to the average untreated (or control) participant. An effect size of 0.0 indicates that the mean of the treatment group is at the 50th percentile of its comparison group. A large effect size of 0.8 indicates that the mean of the treatment group is at the 79th percentile of the comparison group. An effect size of 1.7 indicates that the mean of the treated group is at the 95.5 percentile of the untreated group.

Effect sizes can also be interpreted in terms of the percent of nonoverlap of the treatment group's scores with those of the comparison group. An effecti size of 0.0 indicates that the distribution of scores for the treatment group overlaps completely with the distribution of scores for the comparison group. In other words, there is 0% of nonoverlap. A medium effect size of 0.5 indicates a nonoverlap of 33% in the two distributions.

modified from http://web.uccs.edu/lbecker/Psy590/es.htm