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))

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