What's the probability of observing success in 50 trials? The blue curve represents probabilities under the 'null hypothesis,' when the new treatment is no better than the old. The red curve represents probabilities when the new treatment is better. The shaded area represents the p-value. In this case, the ratio of the probabilities assigned to 22 successes is A divided by B, or 0.21. Valen Johnson, CC BY-SA