[R] Re: point-biserial correlation

Noel Yvonnick noel at univ-lille3.fr
Mon Mar 31 17:07:00 CEST 2003

> 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)

  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)


Yvonnick Noel, PhD.
Department of Psychology
U. of Lille 3

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