[Rd] Problem with pbirthday (PR#7837)
Full_Name: Andy Lynch Version: 1.9.1 OS: Windows Submission from: (NULL) (131.111.86.211) 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 classes. 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). __ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
Re: [Rd] Problem with pbirthday (PR#7837)
On Wed, 4 May 2005 [EMAIL PROTECTED] wrote: 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). Yep, looks like a bug to me. Thanks. I do note that the approximation used in pbirthday is not very good for small n, so fixing this bug still doesn't give the right answer for your example. For small n the best solution is simulation or direct calculation. The point of the pbirthday approximation is that the untrained intuition guesses the probability to be about n(1/classes)^(coincident-1) and the approximation is usually quite good when this is much smaller than the true probability. -thomas __ R-devel@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
