Figure 1
Probability tree diagram showing possible scenarios for finding a compound fit for development with their associated probabilities indicated above and under the arrows. The probability expressions are not cumulative. During the first period, the probability of generating a candidate drug is p, while the probability of not generating one is 1-p. If no drug is discovered during the first interval, a new chance is given during a second interval, which also has a probability of p for success. If no candidate drug is found during the first two periods, a last chance is given during the third interval.