power.t.test              package:stats              R Documentation

Power calculations for one and two sample t tests

Description:

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

Usage:

     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)

Arguments:

       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

Details:

     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.

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):

     Peter Dalgaard.  Based on previous work by Claus Ekstrøm

See Also:

     't.test', 'uniroot'

Examples:

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

