R: Draw a correlation ellipse and two normal curves to...

draw.tetra {psych} | R Documentation

## Draw a correlation ellipse and two normal curves to demonstrate tetrachoric correlation

### 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

    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

### See Also

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

### Examples

    #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))

[Package psych version 1.7.8 Index]