R: Power calculations for one and two sample t tests with...

power_t_test {MESS} | R Documentation

## Power calculations for one and two sample t tests with unequal sample size

### Description

Compute power of test, or determine parameters to obtain target power for equal and unequal sample sizes.

### Usage

    power_t_test(n = NULL, delta = NULL, sd = 1, sig.level = 0.05,
      power = NULL, ratio = 1, sd.ratio = 1, type = c("two.sample",
      "one.sample", "paired"), alternative = c("two.sided", "one.sided"),
      df.method = c("welch", "classical"), strict = FALSE)

### Arguments

`n` | Number of observations (in the smallest group if two groups)
`delta` | True difference in means
`sd` | Standard deviation
`sig.level` | Significance level (Type I error probability)
`power` | Power of test (1 minus Type II error probability)
`ratio` | The ratio n2/n1 between the larger group and the smaller group. Should be a value equal to or greater than 1 since n2 is the larger group. Defaults to 1 (equal group sizes). If ratio is set to NULL (i.e., find the ratio) then the ratio might be smaller than 1 depending on the desired power and ratio of the sd's.
`sd.ratio` | The ratio sd2/sd1 between the standard deviations in the larger group and the smaller group. Defaults to 1 (equal standard deviations in the two groups)
`type` | Type of t test
`alternative` | One- or two-sided test
`df.method` | Method for calculating the degrees of default. Possibilities are welch (the default) or classical.
`strict` | Use strict interpretation in two-sided case

### Details

Exactly one of the parameters `n`, `delta`, `power`, `sd`, `sig.level`, `ratio` `sd.ratio` must be passed as NULL, and that parameter is determined from the others. Notice that the last two have non-NULL defaults so NULL must be explicitly passed if you want to compute them.

If `strict = TRUE` is used, the power will include the probability of rejection in the opposite direction of the true effect, in the two-sided case. Without this the power will be half the significance level if the true difference is zero.

### Value

Object of class `power.htest`, a list of the arguments (including the computed one) augmented with `method` and `note` elements.

### Note

`uniroot` is used to solve power equation for unknowns, so you may see errors from it, notably about inability to bracket the root when invalid arguments are given.

### Author(s)

Claus Ekstrom claus@rprimer.dk

### See Also

`power.t.test`, `power_prop_test`, `power.prop.test`

### Examples

    # Sampling with a ratio of 1:4
    power_t_test(delta=300, sd=450, power=.8, ratio=4)

    # Equal group sizes but different sd's
    # The sd in the first group is twice the sd in the second group
    power_t_test(delta=300, sd=450, power=.8, sd.ratio=.5)

    # Fixed group one size to 50 individuals, but looking for the number of individuals in the
    # second group. Different sd's with twice the sd in the larger group
    power_t_test(n=50, delta=300, sd=450, power=.8, ratio=NULL, sd.ratio=2)

[Package MESS version 0.5.6 Index]