Subject: Re: Incidence of birthdays
Date: Tue 11/10/98 10:31 AM
From: Alex Bogomolny

Dear Bob:

What is the logic behind (3+5)/(1+5) = 3/1? There is none. So, unless you can explain such an operation, it would be strange for you to do that, right?

Can you explain why, in your view, 1/365 x 2/365 has any sense? What properties of probabilities do you use to create such a product? I can explain the backward approach - can you do the same with the "straightforward" one?

The answer to your question is then automatic: do not do anything you can't explain - there is no magic in mathematics. You may get the right result by accident, but normally you will not.

The backward approach is based on the fact that the probability of two INDEPENDENT events is the product of individual probabilities.

What independent events have you in the "straightforward" approach and how, to start with, do you find their individual probabilities?

All the best,
Alexander Bogomolny

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