Pooled Standard Deviation from Two Standard Deviations
The method according to Hedges requires the sample sizes. If only standard deviations are available, the simpler equation provided by Cohen (1988) can be used. If there are more than two groups, SD_M_n_pooled_from_groups should be used.
Note: The use of the names "Cohen" and "Hedges" for the methods can be inconsistent in the literature. It is somewhat unusual because Cohen (1977) outlined both estimators for the pooled standard deviation before Hedges (1981) discussed them.
Result
References
Borenstein, M., Hedges, L.V., Higgins, J.P.T. and Rothstein, H.R. (2009). Converting Among Effect Sizes. In Introduction to Meta-Analysis (eds M. Borenstein, L.V. Hedges, J.P.T. Higgins and H.R. Rothstein). https://doi.org/10.1002/9780470743386.ch7
Cohen, J. (1977). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale, NJ, US: Lawrence Erlbaum Associates, Inc.
Ellis, P.D. (2009), "Effect size equations". https://www.psychometrica.de/effect_size.html Link
Hedges, L. V. (1981). Distribution theory for Glass's estimator of effect size and related estimators.
Journal of Educational Statistics, 6, 107-128.
Difference between Cohen's d and Hedges' g for effect size metrics. Stackoverflow. https://stats.stackexchange.com/q/338043 Link
