• P(D), the pre-test probability or a priori-probability for presence of disease.

  Two important conditional probabilities: diagnostic sensitivity and diagnostic specificity:

• Sensitivity = P(T+/D+); probability to find a positive test result T+ when the disease is present (D+)
• Specificity   = P(T- /D- ); probability to find a negative test result T- when the disease is absent (D-)

Most relevant: Post-test-probability P(D+/T+) indicating the probability for presence of disease (D+), if the test result is positive (T+).