power.t.test {ctest}R Documentation

Power calculations for one and two sample t tests


Compute power of test, or determine parameters to obtain target power.


power.t.test(n=NULL, delta=NULL, sd=1, sig.level=0.05, power=NULL,
             type=c("two.sample", "one.sample", "paired"),
             alternative=c("two.sided", "one.sided"), strict=FALSE)


n Number of observations (per group)
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)
type Type of t test
alternative One- or two-sided test
strict Use strict interpretation in two-sided case


Exactly one of the parameters n, delta, power, sd, and sig.level 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.


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


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.


Peter Dalgaard. Based on previous work by Claus EkstrÝm

See Also

t.test, uniroot


 power.t.test(n=20, delta=1)
 power.t.test(power=.90, delta=1)
 power.t.test(power=.90, delta=1, alt="one.sided")

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