Parametric measures to estimate J. Cohen's effect size (1966) from a single experiment or for a single study in meta-analysis are investigated. The main objective was to examine the principal statistical properties of this effect size--delta--under variance homogeneity, variance heterogeneity with known variance ratios, and for the Behrens-Fisher problem. Derived estimators were compared according to the criteria of their magnitudes, unbiasedness, and mean-square errors. General properties of the derived estimators were examined by means of Monte Carlo results. Results tend to confirm the recommendation of theoretical analysis that two identified estimators, "h" and "d(sub T)," should be used in conducting a meta-analytic study. These estimators better ensure unbiasedness and minimum mean-square error than do others derived. Eight tables and four graphs illustrate study data. (SLD)