power.t.test {ctest} | 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")