# draw.tetra: Draw a correlation ellipse and two normal curves to... In psych: Procedures for Psychological, Psychometric, and Personality Research

## Description

A graphic of a correlation ellipse divided into 4 regions based upon x and y cutpoints on two normal distributions. This is also an example of using the layout function. Draw a bivariate density plot to show how tetrachorics work.

## Usage

 ```1 2 3``` ```draw.tetra(r, t1, t2,shade=TRUE) draw.cor(r=.5,expand=10,theta=30,phi=30,N=101,nbcol=30,box=TRUE, main="Bivariate density rho = ",cuts=NULL,all=TRUE,ellipses=TRUE,ze=.15) ```

## Arguments

 `r` the underlying Pearson correlation defines the shape of the ellipse `t1` X is cut at tau `t2` Y is cut at Tau `shade` shade the diagram (default is TRUE) `expand` The relative height of the z axis `theta` The angle to rotate the x-y plane `phi` The angle above the plane to view the graph `N` The grid resolution `nbcol` The color resolution `box` Draw the axes `main` The main title `cuts` Should the graphic show cuts (e.g., cuts=c(0,0)) `all` Show all four parts of the tetrachoric `ellipses` Draw a correlation ellipse `ze` height of the ellipse if requested

## Details

A graphic demonstration of the `tetrachoric` correlation. Used for teaching purposes. The default values are for a correlation of .5 with cuts at 1 and 1. Any other values are possible. The code is also a demonstration of how to use the `layout` function for complex graphics using base graphics.

## Author(s)

William Revelle

`tetrachoric` to find tetrachoric correlations, `irt.fa` and `fa.poly` to use them in factor analyses, `scatter.hist` to show correlations and histograms.

## Examples

 ```1 2 3 4 5 6 7 8``` ```#if(require(mvtnorm)) { #draw.tetra(.5,1,1) #draw.tetra(.8,2,1)} else {print("draw.tetra requires the mvtnorm package") #draw.cor(.5,cuts=c(0,0))} draw.tetra(.5,1,1) draw.tetra(.8,2,1) draw.cor(.5,cuts=c(0,0)) ```

### Example output    ```
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psych documentation built on June 19, 2021, 1:06 a.m.