Events A and B are independent if the occurrence of either does not affect the probability of the other occurring. Otherwise, A and B are dependent. For independent events, the probability that they both occur is given by the product law Pr(A ∩ B) = Pr(A) Pr(B). This is equivalent to the conditional probability Pr(A | B) equalling Pr(A), provided Pr(B)≠0.

For example, when an unbiased coin is tossed twice, the probability of obtaining ‘heads’ on the first toss is $\frac{1}{2}$, and the probability of obtaining ‘heads’ on the second toss is $\frac{1}{2}$. These two events are independent. So the probability of obtaining ‘heads’ on both tosses is equal to $\frac{1}{2}\times \frac{1}{2}=\frac{1}{4}$.