size.test.lc.ancova {statpsych}

Sample size for a mean linear contrast test in an ANCOVA

Description

Computes the sample size for each group (assuming equal sample sizes) required to test a linear contrast of population means in an ANCOVA model with desired power. 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 all pairwise comparisons and for 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.

Usage

size.test.lc.ancova(alpha, pow, evar, es, s, d, v)

Arguments

alpha

alpha level for hypothesis test

pow

desired power

evar

planning value of within-group (error) variance

es

planning value of linear contrast

s

number of covariates

d

largest standardized mean difference for all covariates

v

vector of between-subjects contrast coefficients

Value

Returns the required sample size for each group

Examples

v <- c(.5, .5, -1)
size.test.lc.ancova(.05, .9, 1.37, .7, 1, 0, v)

# Should return:
# Sample size per group
#                    47