where 1 ≤ x, y ≤ 4. We calculate the number of elements in eq 4, N , and then use eq 2 to find the joint probability of forming active compounds for pairs of building block positions (Figure ). The joint probabilities reveal that typically for disynthon combinations, or a pair of building block positions, increasing the P(active) of either building block increases the probability of forming an active compound (Figure -C, Table ). Furthermore, we find that high P(active) building blocks can be used to rescue activity when combined with building blocks with lower P(active). The higher the P(active) of a building block in one position is, the lower the P(active) in another needs to be to achieve the same probability of forming an active compound. There is a noticeable increase in the probability of forming an active compound when the building blocks in both positions have P(active) values greater than 0.20 (bin 1) (Figure ).