powertools (version 1.0.0)

ttest.2samp: Power calculation for two-sample t test

Description

This function computes power and sample size for a two-sample t test. Unequal sample sizes and/or unequal variances in the two groups are allowed. Can solve for power, n1, n.ratio, sd1, sd.ratio, delta or alpha.

Usage

ttest.2samp(
  n1 = NULL,
  n.ratio = 1,
  delta = NULL,
  sd1 = 1,
  sd.ratio = 1,
  df.method = c("welch", "classical"),
  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.

df.method

Method for calculating the degrees of freedom: "welch" (default) or "classical", which is n1+n2-2.

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. For an equivalence test for two means, see the sampleN.TOST function from the PowerTOST package.

Examples

ttest.2samp(n1 = 50, delta = 2, sd1 = 5, sides = 1)
ttest.2samp(n1 = NULL, n.ratio = 2, delta = 0.5, sd1 = 1, power = 0.8, sides = 2)
ttest.2samp(n1 = 49, n.ratio = 2, delta = 0.5, sd1 = 1, power = NULL, sides = 2)
ttest.2samp(n1 = 25, n.ratio = 3, delta = 3, sd1 = 4, sd.ratio = 1.5, alpha = 0.025, sides = 1)
ttest.2samp(n1 = NULL, delta = 0.5, sd1 = 1, power = 0.8, sides = 2)