[Rd] Problem with pbirthday (PR#7837)

andy.lynch at phpc.cam.ac.uk andy.lynch at phpc.cam.ac.uk
Wed May 4 15:07:06 CEST 2005

Full_Name: Andy Lynch
Version: 1.9.1
OS: Windows
Submission from: (NULL) (

As I understand it, pbirthday(n,c,k) gives the approximate probability that we
see a class with k coicident people in it when n people are sorted into c

so the command 

> pbirthday(4,classes=3,coincident=4) 

should give the approximate probability that when four people fall into three
classes, all four end up in the same class. A probability that is clearly lower
than the presently returned value of 1. 

It seems to me that the line in the function 

if (n > classes) return(1) 

is only relevant to the default case of coincident = 2. (Naturally, if there are
more people than classes, then at least one class must contain 2 people).

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