# prec_mean: Sample size or precision for a mean In presize: Precision Based Sample Size Calculation

## Description

`prec_mean` returns the sample size or the precision for the provided mean and standard deviation.

## Usage

 `1` ```prec_mean(mu, sd, n = NULL, conf.width = NULL, conf.level = 0.95, ...) ```

## Arguments

 `mu` mean. `sd` standard deviation. `n` number of observations. `conf.width` precision (the full width of the confidence interval). `conf.level` confidence level. `...` other arguments to uniroot (e.g. `tol`).

## Details

Exactly one of the parameters `n` or `conf.width` must be passed as NULL, and that parameter is determined from the other.

The precision is defined as the full width of the confidence interval. The confidence interval calculated as t(n - 1) * sd / sqrt(n), with t(n-1) from the t-distribution with n-1 degrees of freedom.

`uniroot` is used to solve `n`.

## Value

Object of class "presize", a list with `mu` mean, `sd` standard deviation, `n` sample size, `conf.width` precision (the width of the confidence interval), `lwr` lower bound of confidence interval, `upr` upper bound of confidence interval, augmented with method and note elements.

## Examples

 ```1 2 3 4``` ```# mean of 5, SD of 2.5, whats the confidence interval width with 20 participants? prec_mean(mu = 5, sd = 2.5, n = 20) # mean of 5, SD of 2.5, how many participants for CI width of 2.34? prec_mean(mu = 5, sd = 2.5, conf.width = 2.34) # approximately the inverse of above ```

presize documentation built on April 9, 2021, 1:05 a.m.