[Rd] Problem with pbirthday (PR#7837)

Thomas Lumley tlumley at u.washington.edu
Wed May 4 16:50:00 CEST 2005

On Wed, 4 May 2005 andy.lynch at phpc.cam.ac.uk 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
and the approximation is usually quite good when this is much smaller than 
the true probability.


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