Classical Probability: The "Fair Game" Definition
Imagine you roll a fair six-sided die. Before it lands, you know there are exactly six possible outcomes — 1, 2, 3, 4, 5, or 6 — and you have no reason to believe any one face is more likely than another. That gut feeling of "all outcomes are equally likely" is the entire foundation of classical probability.
The Intuition
Classical probability was born from games of chance — dice, coins, cards. In these settings, the physical symmetry of the objects (a balanced die, a fair coin) guarantees that no outcome is favoured. So the probability of an event is simply:
Number of ways the event can happen, divided by the total number of possible outcomes.
If you want the chance of rolling an even number on a die, count the evens: 2, 4, 6 — that's 3 ways. Total outcomes: 6. So probability = 3/6=1/2.
This is the "counting" approach. It works beautifully when the underlying experiment is symmetric and finite.
The Precise Statement
P(E)=Total number of equally likely outcomesNumber of outcomes favourable to event E
This is called the classical definition (or a priori definition) of probability. It was formalised by Pierre-Simon Laplace in the 18th century.
Three conditions must hold for this definition to apply:
- Finite sample space — there are only a fixed, countable number of possible outcomes.
- Equally likely outcomes — each outcome has the same chance of occurring (the "fairness" condition).
- Mutually exclusive outcomes — no two outcomes can happen at the same time.
The biggest mistake students make is applying classical probability to situations where outcomes are not equally likely. For example: "I can either pass or fail the exam — two outcomes, so probability of passing is 1/2." That's nonsense, because passing and failing are not equally likely. The die works only because the die is fair.
A Simple Example
Problem: A bag contains 3 red marbles and 2 blue marbles. You pick one marble at random. What is the probability it is red?
Step 1 — Identify the sample space: There are 5 marbles total. If the marbles are physically identical except for colour, and you pick without looking, each marble is equally likely to be chosen. So total outcomes = 5.
Step 2 — Count favourable outcomes: 3 marbles are red. So favourable outcomes = 3.
Step 3 — Apply the formula:
P(red)=53
That's it. No deeper theory needed for this case.
When Classical Probability Fails
Classical probability cannot handle: …