# betaGetShapes: Convert beta mean and precision parameters to two shape... In kgoldfeld/simstudy: Simulation of Study Data

## Description

Convert beta mean and precision parameters to two shape parameters

## Usage

 `1` ```betaGetShapes(mean, precision) ```

## Arguments

 `mean` The mean of a beta distribution `precision` The precision parameter (phi) of a beta distribution

## Details

In simstudy, users specify the beta distribution as a function of two parameters - a mean and precision, where 0 < mean < 1 and precision > 0. In this case, the variance of the specified distribution is (mean)*(1-mean)/(1+precision). The base R function rbeta uses the two shape parameters to specify the beta distribution. This function converts the mean and precision into the shape1 and shape2 parameters.

## Value

A list that includes the shape parameters of the beta distribution

## Examples

 ```1 2 3 4 5 6 7 8 9``` ```set.seed(12345) mean = 0.3; precision = 1.6 rs <- betaGetShapes(mean, precision) c(rs\$shape1, rs\$shape2) vec <- rbeta(1000, shape1 = rs\$shape1, shape2 = rs\$shape2) (estMoments <- c(mean(vec), var(vec))) (theoryMoments <- c(mean, mean*(1-mean)/(1+precision))) (theoryMoments <- with(rs, c(shape1/(shape1 + shape2), (shape1*shape2) / ((shape1 + shape2)^2*(1 + shape1 + shape2))))) ```

kgoldfeld/simstudy documentation built on Dec. 23, 2018, 5:33 a.m.