Sample size for a 2-group ANCOVA hypothesis test

Computes the sample size for each group required to test a mean difference in an ANCOVA model with desired power in a 2-group design. In a nonexperimental design, the sample size is affected by the magnitude of covariate mean differences across groups. The covariate mean differences can be approximated by specifying the largest standardized covariate mean difference across of all covariates. In an experiment, this standardized mean difference is set to 0. Set the error variance planning value to the largest value within a plausible range for a conservatively large sample size.

For more details, see Section 2.29 of Bonett (2021, Volume 2)

Usage

size.test.ancova2(alpha, pow, evar, es, s, d, R)

Arguments

alpha

alpha level for hypothesis test

pow

desired power

evar

planning value of within-group (error) variance

es

planning value of mean difference

s

number of covariates

d

largest standardized mean difference of all covariates

R

n2/n1 ratio

Value

Returns the required sample size for each group

References

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

Examples

size.test.ancova2(.05, .9, 1.37, .7, 1, 0, 1)
#>  n1 n2
#>  61 61

# Should return:
#  n1 n2
#  61 61

size.test.ancova2(.05, .9, 1.37, .7, 1, 0, 2)
#>  n1 n2
#>  47 94

# Should return:
#  n1 n2
#  47 94

size.test.ancova2(.05, .9, 1.37, .7, 1, .5, 1)
#>  n1 n2
#>  65 65

# Should return:
#  n1 n2
#  65 65