Computes the sample size for each group required to estimate a mean difference in a 2-group ANCOVA model with desired confidence interval precision. 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 of all covariates. In an experiment, this standardized mean difference should be set to 0. Set the error variance planning value to the largest value within a plausible range for a conservatively large sample size.
size.ci.ancova2(alpha, evar, s, d, w, R)Returns the required sample size for each group
alpha level for 1-alpha confidence
planning value of within group (error) variance
number of covariates
largest standardized mean difference of all covariates
desired confidence interval width
ratio of n2/n1
size.ci.ancova2(.05, 1.37, 1, 0, 1.5, 1)
# Should return:
# n1 n2
# 21 21
size.ci.ancova2(.05, 1.37, 1, 0, 1.5, 2)
# Should return:
# n1 n2
# 16 32
size.ci.ancova2(.05, 1.37, 1, .75, 1.5, 1)
# Should return:
# n1 n2
# 24 24
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