THE LAWS OF PROBABILITY

THE LAWS OF PROBABILITY

1. Here's a new mnemonic rhyme for the formula to determine the likely outcome of two alternative events:

" For either/or, add both together
minus both at once (if ever)... "

ie. P(A or B) = P(A) + P(B) - P(A and B)

The "addition" formula governs mutually exclusive events such as "heads or tails" where P(A and B) is always zero (ie. impossible), and the occurrence of single independent outcomes such as "rich or tall".

2. For the probability of joint independent events such as "rich and tall" occurring at the same time:

" And if you need to both discover,
multiply one times the other... "

ie. P(A and B) = P(A) * P(B)

3. For conditional probability, where a likelihood of B depends upon (or is altered by) the outcome of A:

" Only when affects position,
multiply by changed condition... "

ie. P(A and B) = P(A) * P(B given A)

The "mathematical poetry" is by Peter Hobbs, but similar rhymes are likely to have been made up by far better mathematicians in the past!