1. in an analysis of variance or an analysis of covariance, an effect size index that represents the standard deviation of a set of group means divided by the average standard deviation of scores across the set of groups. Its value ranges from 0 to 1, with small, medium, and large effect size values suggested as .10, .25, and .40, respectively.

2. in a table or distribution, symbol for frequency.

3. in vision science, symbol for focal length.