SDp_from_SD {metaHelper}

Pooled Standard Deviation from Two Standard Deviations

Description

Calculates the pooled standard deviation.

Usage

SDp_from_SD(SD1, SD2, n1 = NA, n2 = NA, method = "hedges")

Arguments

SD1

standard deviation of group 1

SD2

standard deviation of group 2

n1

sample size of group 1

n2

sample size of group 2

method

the method ("hedges", "cohen") that should be used to calculate the SD. Method "hedges" requires sample sizes. The "cohen" method uses a simplified method by and does not rely on sample sizes.

Details

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.

Value

Pooled standard deviation

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". 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. Link

See Also

SD_within_from_SD_r() for matched groups

Examples

# Standard deviation according to Cohen:
SDp_from_SD(2, 3, method = "cohen")

# Standard deviation according to Hedges needs sample sizes:
SDp_from_SD(2, 3, 50, 50)