# power.t.test: Power calculations for one and two sample t tests with...

### Description

Compute power of test, or determine parameters to obtain target power for equal and unequal sample sizes.

### Usage

 ```1 2 3 4``` ```power.t.test(n = NULL, delta = NULL, sd = 1, sig.level = 0.05, power = NULL, ratio = 1, sd.ratio = 1, type = c("two.sample", "one.sample", "paired"), alternative = c("two.sided", "one.sided"), df.method = c("welch", "classical"), 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) `ratio` The ratio n2/n1 between the larger group and the smaller group. Should be a value equal to or greater than 1 since n2 is the larger group. Defaults to 1 (equal group sizes) `sd.ratio` The ratio sd2/sd1 between the standard deviations in the larger group and the smaller group. Defaults to 1 (equal standard deviations in the two groups) `type` Type of t test `alternative` One- or two-sided test `df.method` Method for calculating the degrees of default. Possibilities are welch (the default) or classical. `strict` Use strict interpretation in two-sided case

### Details

Exactly one of the parameters `n`, `delta`, `power`, `sd`, `sig.level`, `ratio` `sd.ratio` 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)

Claus Ekstrom claus@rprimer.dk

`power.prop.test`

### Examples

 `1` ```power.t.test(delta=300, sd=450, power=.8, ratio=4) ```

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