The probabilities of a test are represented by I for infected; U, uninfected; D, infection detected; and N, no infection detected. What is the symbolic way to represent the probability of a true positive?

A true positive is when a person who is infected receives a positive test (someone whose test comes back positive but who doesn't really have the infection would be called a false positive).

A symbolic way to represent the probability of a true positive would be the infection detected divided by the total of all the options:

D / (I + U + D + N)