Classical Probability
Probability of an event with equally likely outcomes.
Formula
Variables
Example
Rolling a 6: P = 1/6
How it works
Classical probability is used when every possible outcome of an experiment is equally likely, so each outcome has the same chance of happening. In that case, the chance of an event is found by counting how many outcomes make the event happen and dividing by the total number of possible outcomes. This works because equal likelihood lets us treat each outcome as one fair share of the whole sample space.
When to use it
Use this formula for fair or symmetric situations like dice, coins, cards, spinners, or choosing one item at random from a complete set. It is most appropriate when you can clearly list all outcomes and each one has the same chance.
Worked example, step by step
Example: A fair coin is flipped once. What is the probability of getting heads? Step 1: Count the favorable outcomes for heads = 1 Step 2: Count the total possible outcomes = 2 (heads, tails) Step 3: Substitute into the formula: P(heads) = favorable outcomes / total outcomes = 1/2 Final answer: P(heads) = 1/2
Common mistakes to avoid
- Counting outcomes that are not equally likely and using the formula anyway
- Forgetting to include every possible outcome in the total
- Mixing up favorable outcomes with total outcomes and reversing the fraction
Did You Know?
This definition assumes all outcomes are equally likely.
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