powertools (version 1.0.0)

ztest.2samp: Power calculation for two-sample z test

Description

This function performs power and sample size calculations for a two-sample z test. which is analogous to a two-sample t test but with the variances assumed to be known. The function allows for unequal sample sizes and/or variances in the two groups. This function is provided largely for pedagogical purposes; in general, for real studies, the two-sample t test procedure should be used.

Usage

ztest.2samp(
  n1 = NULL,
  n.ratio = 1,
  delta = NULL,
  sd1 = 1,
  sd.ratio = 1,
  alpha = 0.05,
  power = NULL,
  sides = 2,
  v = FALSE
)

Value

A list of the arguments (including the computed one).

Arguments

n1

The sample size for group 1.

n.ratio

The ratio n2/n1 between the sample sizes of two groups; defaults to 1 (equal group sizes).

delta

DeltaA (true difference mu1 - mu2) - Delta0 (difference under the null). For a noninferiority or superiority by a margin test, the margin should be subtracted, that is, delta = DeltaA - Delta0 - margin.

sd1

The standard deviation for group 1; defaults to 1 (equal standard deviations in the two groups).

sd.ratio

The ratio sd2/sd1 between the standard deviations of the two groups.

alpha

The significance level (type 1 error rate); defaults to 0.05.

power

The specified level of power.

sides

Either 1 or 2 (default) to specify a one- or two- sided hypothesis test.

v

Either TRUE for verbose output or FALSE (default) to output computed argument only.

Details

For a noninferiority or superiority by a margin test, the margin should be subtracted as part of the specification of delta and a one-sided test should be used. See Crespi (2025) for specific guidance.

Examples

ztest.2samp(n1 = NULL, n.ratio = 1, delta = 0.5, sd1 = 1, power = 0.8, sides = 2)