Mass screening represents a method of minimizing sequelae of some of the inheritable metabolic diseases by enabling early diagnosis and treatment. Conditional probability theory can be used to evaluate the effectiveness of screening programs and to plan programs which are appropriate to local populations. Depending on the rarity of a disease in a population, the probability that a positive test indicates the presence of the disease is low. However, by modifying the test procedure and using multiple tests, the probability of true-positives can be increased to practical levels.
Sproul A. Influence of Conditional Probability on the Effectiveness of Mass-Screening Programs. JAMA. 1966;196(4):315-318. doi:10.1001/jama.1966.03100170057017