If you have n balls, all of different colours, and draw one t times, 
at random, 
putting it back afterwards, what is the probability of drawing the 
red ball r times (out of the t times, order without importance)?

Hi, mariane0-ga:

This is a simple binomial probability.

If we ask for the chance of getting the red ball exactly r times out
of t draws, then the answer is:

C(t,r) * (1/n)^r * (1 - 1/n)^(t-r)

where C(t,r) = t!/(r!(t-r)!) is the number of combinations of t things
taken r at a time (the familiar binomial coefficient).

Please let me know if my answer requires further clarification.

regards, mathtalk-ga

Great, just what I needed to know to compare classifier results. When I asked 
this question to people who know about classifiers, they didn't know... :-).
Thank you very much.