Sensitivity | True positive rate (positive in disease) | How good is this test at picking up people who have the condition? | a/ (a+c) |

Specificity | True negative rate (negative in health) | How good is this test at correctly excluding people without the condition? | d/ (b+d) |

Positive predictive value | Post-test probability of a positive test | If a person tests positive, what is the probability that he or she has the condition? | a/ (a+b) |

Negative predictive value | Post-test probability of a negative test | If a person tests negative, what is the probability that he or she does not have the condition? | d/ (c+d) |

Accuracy | — | What proportion of all tests have given the correct result? (true positives and true negatives as a proportion of all results) | (a+d)/ (a+b+c+d) |

Likelihood ratio of a positive test | — | How much more likely is a positive test to be found in a person with the condition than in a person without it? | sensitivity/ (l-specificity) |

Likelihood ratio of a negative test | — | How much more likely is a negative test to be found in a person without the condition than in a person with it | (l-sensitivity)/specificity? |