Probability Formulas
P(A) = Favourable Outcomes / Total Outcomes
| Event | Formula |
|---|---|
| Single event | P(A) = f / n |
| Complement | P(not A) = 1 − P(A) |
| A and B (independent) | P(A∩B) = P(A) × P(B) |
| A or B (mutually exclusive) | P(A∪B) = P(A) + P(B) |
| A or B (general) | P(A∪B) = P(A) + P(B) − P(A∩B) |
Example
Coin Flip
Probability of heads: P = 1/2 = 0.5 = 50%
Probability of 3 heads in a row: P = 0.5 × 0.5 × 0.5 = 12.5%
What is probability?
Probability measures how likely an event is to occur, expressed as a number between 0 (impossible) and 1 (certain). It can also be expressed as a percentage.
What is conditional probability?
P(A|B) is the probability of A given that B has already occurred: P(A|B) = P(A∩B) / P(B).