Specificity is defined as which of the following?

Specificity is defined as which of the following?

• A. Probability a test is negative given no disease
• B. Probability a test is positive given disease
• C. Probability a test is negative given disease
• D. Probability a test is positive given no disease

Answer: (A)

Specificity measures how well a test identifies people who do not have the disease. It is the probability that the test result is negative when the person does not have the disease. A highly specific test yields few false positives, so a negative result in someone without disease is reliable. For example, if 100 disease-free individuals take the test and 95 test negative, the specificity is 95%. The other concepts listed correspond to different ideas: sensitivity is the probability the test is positive given disease; the probability the test is negative given disease is the false negative rate; and the probability the test is positive given no disease is the false positive rate (1 − specificity).

Specificity measures how well a test identifies people who do not have the disease. It is the probability that the test result is negative when the person does not have the disease. A highly specific test yields few false positives, so a negative result in someone without disease is reliable. For example, if 100 disease-free individuals take the test and 95 test negative, the specificity is 95%. The other concepts listed correspond to different ideas: sensitivity is the probability the test is positive given disease; the probability the test is negative given disease is the false negative rate; and the probability the test is positive given no disease is the false positive rate (1 − specificity).