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

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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

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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

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#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))

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