# Probability Notation Help

1. Nov 5, 2012

### CaseyJRichard

In probability, P(A) is the probability of event A occurring.

What is O(A), in the same context? I've tried looking this up, but I can't find the definition/explanation of this notation anywhere.

For an example, see the definition of "Bayes' rule".

2. Nov 5, 2012

### Ibix

Do you mean the definition at Wikipedia? It defines O in the fourth equation under "Derivation".

3. Nov 15, 2012

### CaseyJRichard

It's not really defining O there, is it? It's defining an equation that uses O, sure, but it still doesn't explain what the notation "O" represents. But maybe I just don't understand the equation.

Regardless, the answer I was looking for is:

O(A) is the "Odds" of event A occurring (versus event A not occurring), whereas P(A) is the "Probability" of event A occurring. (i.e., the number of outcomes where A occurs over the total number of outcomes.)