size.ci.ancova2: Sample size for a 2-group ANCOVA confidence interval

In statpsych: Statistical Methods for Psychologists

Sample size for a 2-group ANCOVA confidence interval

Description

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.

Usage

size.ci.ancova2(alpha, evar, s, d, w, R)

Arguments

alpha	alpha level for 1-alpha confidence
evar	planning value of within group (error) variance
s	number of covariates
d	largest standardized mean difference of all covariates
w	desired confidence interval width
R	ratio of n2/n1

Value

Returns the required sample size for each group

Examples

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
 

statpsych documentation built on April 4, 2025, 3:20 a.m.