# code to illustrate various aspects of plotting functions in R

p <- seq( 0.001, 0.999, length = 500 )

plot( p, log( p / ( 1 - p ) ), type = 'l', lwd = 2,
  ylab = 'logit( p ) = log( p / ( 1 - p ) )' )

text( 0.4, 4.0, 'this is the logistic or logit transformation;' )

text( 0.4, 3.25, 'it maps probabilities onto the whole real line' )

lines( p, 4 * p - 2, lty = 2, lwd = 2, col = 'red' )

text( 0.6, -4.0, "it's approximately linear for p in ( 0.3, 0.7 )" )

# use this code to make a PDF file to incorporate into your test

pdf( 'logit-plot.pdf' )

plot( p, log( p / ( 1 - p ) ), type = 'l', lwd = 2,
  ylab = 'logit( p ) = log( p / ( 1 - p ) )' )

text( 0.4, 4.0, 'this is the logistic or logit transformation;' )

text( 0.4, 3.25, 'it maps probabilities onto the whole real line' )

lines( p, 4 * p - 2, lty = 2, col = 'red' )

text( 0.6, -4.0, "it's approximately linear for p in ( 0.3, 0.7 )" )

dev.off( )