Bayes’ Theorem

Maths: Statistics for machine learning

2 min read

Published Oct 22 2025, updated Aug 17 2026

Machine LearningMathsNumPyPandasPythonStatistics

Bayes’ Theorem describes how to update the probability of a hypothesis when given new evidence.

It tells us:

“How likely is something (a hypothesis) to be true, given we’ve seen some data (evidence)?”

Highlevel formula

Posterior = (Likelihood × Prior) / Evidence

It starts with what you already believe (prior) and updates it using new data (likelihood).

Real-World Example — Medical Test

Let’s say:

1% of people have a certain disease (P(Disease) = 0.01)
A test is 99% accurate:

If you have the disease, it’s positive 99% of the time (P(Pos|Disease) = 0.99)
If you don’t, it’s negative 99% of the time (P(Neg|NoDisease) = 0.99)

Now, you take the test and it comes back positive.
What’s the probability you actually have the disease? (P(Disease|Pos))

Python example:

# Given valuesP_disease = 0.01P_no_disease = 1 - P_diseaseP_pos_given_disease = 0.99P_pos_given_no_disease = 0.01# Calculate total probability of testing positiveP_positive = (P_pos_given_disease * P_disease) + (P_pos_given_no_disease * P_no_disease)# Apply Bayes' theoremP_disease_given_positive = (P_pos_given_disease * P_disease) / P_positiveprint(f"Probability of disease given positive test: {P_disease_given_positive:.2%}")

Output:

Probability of disease given positive test: 50.00%

Even with a 99% accurate test, if the disease is rare, a positive result only means a 50% chance you actually have it.

Intuition

The prior captures what you already believe (disease is rare).
The likelihood shows how well the evidence fits that belief (positive test).
The posterior gives your updated belief after seeing the new data.

This “belief updating” is what powers Bayesian inference — the foundation of probabilistic learning.