# draw.tetra

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

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.

- Keywords
- multivariate, hplot

##### 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.

##### 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

```
# NOT RUN {
#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))
# }
```

*Documentation reproduced from package psych, version 1.8.12, License: GPL (>= 2)*