Sample size for a 2-group standardized mean difference confidence interval

Computes the sample size per group required to estimate two types of population standardized mean differences (unweighted standardizer and single group standardizer) with desired confidence interval precision in a 2-group design. Set the standardized mean difference planning value to the largest value within a plausible range for a conservatively large sample size. Set R = 1 for equal sample sizes. For unequal sample sizes, this function assumes approximately equal population variances.

For more details, see Section 2.13 of Bonett (2021, Volume 1)

Usage

size.ci.stdmean2(alpha, d, w, R)

Arguments

alpha

alpha level for 1-alpha confidence

d

planning value of standardized mean difference

w

desired confidence interval width

R

n2/n1 ratio

Value

Returns the required sample size per group for each standardizer

References

Bonett DG (2009). “Estimating standardized linear contrasts of means with desired precision.” Psychological Methods, 14(1), 1–5. ISSN 1939-1463, doi:10.1037/a0014270 .

Bonett DG (2021). Statistical Methods for Psychologists https://dgbonett.sites.ucsc.edu/.

Examples

size.ci.stdmean2(.05, 1.0, .5, 1)
#>                             n1  n2
#> Unweighted standardizer:   139 139
#> Single group standardizer: 154 154

# Should return:
#                              n1  n2
# Unweighted standardizer:    139 139
# Single group standardizer:  154 154

size.ci.stdmean2(.05, 1.0, .5, 2)
#>                             n1  n2
#> Unweighted standardizer:   104 208
#> Single group standardizer: 116 232

# Should return:
#                              n1  n2
# Unweighted standardizer:    104 208
# Single group standardizer:  116 232