> has anyone written a package/function in R for computing a point-
> biserial resp. biserial correlation?
Note that the point-biserial correlation is nothing but the standard
correlation coefficient when one of the variables is dichotomous, so that
cor(.) is OK.
The biserial is different and includes a correction for the so-called "point
of dichotomy". The following should work (translating a formula found in a
psychometric manual) :
cor.biserial = function(x,y)
{
stopifnot(is.factor(x))
stopifnot(length(levels(x))==2)
stopifnot(length(x)==length(y))
N = length(y)
# Success / Failure frequencies
f = table(x)/length(x)
# Means of success/failure groups on the global score
m = tapply(y,x,mean)
# Variance of the global score
Sy = sqrt(var(y)*(N-1)/N)
# Biserial correlation
# Be cautious in interpreting the sign :
# depends upon the ordering of levels(x)
((m[1]-m[2])/Sy)*(f[1]*f[2]/dnorm(f[1]-.5))
}
Yvonnick Noel, PhD.
Department of Psychology
U. of Lille 3
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