R: Power calculations for t-tests of means (one sample, two...

pwr.t.test {pwr} | R Documentation

## Power calculations for t-tests of means (one sample, two samples and paired samples)

### Description

Compute power of tests or determine parameters to obtain target power (similar to power.t.test).

### Usage

    pwr.t.test(n = NULL, d = NULL, sig.level = 0.05, power = NULL,
        type = c("two.sample", "one.sample", "paired"),
        alternative = c("two.sided", "less", "greater"))

### Arguments

`n` | Number of observations (per sample)
`d` | Effect size (Cohen's d) - difference between the means divided by the pooled standard deviation
`sig.level` | Significance level (Type I error probability)
`power` | Power of test (1 minus Type II error probability)
`type` | Type of t test : one- two- or paired-samples
`alternative` | a character string specifying the alternative hypothesis, must be one of "two.sided" (default), "greater" or "less"

### Details

Exactly one of the parameters 'd','n','power' and 'sig.level' must be passed as NULL, and that parameter is determined from the others. Notice that the last one has non-NULL default so NULL must be explicitly passed if you want to compute it.

### 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)

Stephane Champely <champely@univ-lyon1.fr> but this is a mere copy of Peter Dalgaard work (power.t.test)

### References

Cohen, J. (1988). Statistical power analysis for the behavioral sciences (2nd ed.). Hillsdale,NJ: Lawrence Erlbaum.

### See Also

power.prop.test

### Examples

    ## One sample (power)
    ## Exercise 2.5 p. 47 from Cohen (1988)
    pwr.t.test(d=0.2,n=60,sig.level=0.10,type="one.sample",alternative="two.sided")

    ## Paired samples (power)
    ## Exercise p. 50 from Cohen (1988)
    d<-8/(16*sqrt(2*(1-0.6)))
    pwr.t.test(d=d,n=40,sig.level=0.05,type="paired",alternative="two.sided")

    ## Two independent samples (power)
    ## Exercise 2.1 p. 40 from Cohen (1988)
    d<-2/2.8
    pwr.t.test(d=d,n=30,sig.level=0.05,type="two.sample",alternative="two.sided")

    ## Two independent samples (sample size)
    ## Exercise 2.10 p. 59
    pwr.t.test(d=0.3,power=0.75,sig.level=0.05,type="two.sample",alternative="greater")

[Package pwr version 1.2-2 Index]