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.

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 .

Examples

size.ci.stdmean2(.05, .75, .5, 1)
#>                             n1  n2
#> Unweighted standardizer:   132 132
#> Single group standardizer: 141 141

# Should return:
#                              n1  n2
# Unweighted standardizer:    132 132
# Single group standardizer:  141 141

size.ci.stdmean2(.05, .75, .5, 2)
#>                             n1  n2
#> Unweighted standardizer:    99 198
#> Single group standardizer: 106 212

# Should return:
#                              n1  n2
# Unweighted standardizer:     99 198
# Single group standardizer:  106 212