pwr (version 1.0)

pwr.t.test: 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 (same as power.t.test except for the definition of the effect size).

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", "one.sided"))

Arguments

n

Number of observations (per sample)

d

Effect size

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

One- or two-sided test

Value

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

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.

References

J. Cohen (1988) Statistical power analysis for the behavioral scientist. Lawrence Erlbaum Associates, publishers.

See Also

power.t.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")

## One sample (sample size)
## Exercise 2.12 p. 61 from Cohen (1988)
pwr.t.test(d=0.1,power=0.90,sig.level=0.01,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")

## Paired samples (sample size)
## Exercise 2.15 p. 66 from Cohen (1988)
d<-0.5/sqrt(2*(1-0.65))
pwr.t.test(d=d,power=0.95,sig.level=0.01,type="one.sample",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 from Cohen (1988)
pwr.t.test(d=0.3,power=0.75,sig.level=0.05,type="two.sample",alternative="one.sided")