Binomial Distribution

The binomial distribution counts how many successes you get in a fixed number of independent yes/no trials that each succeed with the same probability p. It answers questions like “out of 100 coin flips, what is the chance of getting exactly 60 heads?”

It is one of the workhorses of A/B testing and quality control, anywhere you tally successes out of n attempts. Its mean is n times p (the number you expect), and as n grows its shape starts to look like a normal curve, foreshadowing the central limit theorem.