phi2poly: Convert a phi coefficient to a polychoric correlation
Description
Given a phi coefficient (a Pearson r calculated on two dichotomous variables), and the marginal frequencies (in percentages), what is the corresponding estimate of the polychoric correlation?
Given a two x two table of counts
lll{
a b
c d
}
The phi coefficient is (a - (a+b)*(a+c))/sqrt((a+b)(a+c)(b+d)(c+c)).
This function reproduces the cell entries for specified marginals and then calls John Fox's polychor function.
Usage
phi2poly(ph, cp, cc)
Arguments
ph
phi
cp
probability of the predictor -- the so called selection ratio
cc
probability of the criterion -- the so called success rate.
Value
a polychoric correlation
Details
Uses John Fox's polycor package, which in turn requires the mvtnorm package