### GIGAMIND

Folder:
113 Math
File:
113.038.10 Probability - Bayes Theorem

# Bayes Theorem

The probability of event A given some new evidence B
- $$P(A | B) = \frac{P(B | A) * P(A)}{P(B)}$$
- Denominator can be read as "total probability of B":

• Use trimmed trees if you need a visual. E.g.
• Given 2 coins, one fair, one with 2 sides of heads
• What is the likelihood of a heads from the fair coin?
• Flip 1
• Fair coin = 2 outcomes, 1 heads, 1 tails
• Unfair coin = 1 outcome, 2 heads
• If the result is heads, we remove tails from the outcome set b/c we know it could only happen with the fair coin.
• Then we take our one result and divide by the total number of outcomes left, in this case 3.
• So the probability of getting a fair heads in this case is 1/3

##### Tags:
• statistics
• probabilities