To provide an example, let’s suppose a physician is attempting to determine whether a patient has a disease that occurs in 1% of a defined pa-tient population. When the test is performed on patients with the disease, it yields a positive test result indicating the presence of the disease in 90% of the patients (sensi-tivity equals .90). When the test is performed on patients without the disease, it correctly identifies 98% of those patients as disease free (specificity equals .98). An equa-tion based on Bayes Theorem can be used to calculate the probability that a patient with a positive test result actually has the disease. The simple equation for calcu-lating this probability is: