Likelihood Ratio Slide Rule – Evidence-Based Diagnosis

This slide rule calculates the post-test probability of disease P(D+|r), given the pre-test probability P(D+) and the likelihood ratio of the test result LR(r). See Chapters 2 and 3 of Evidence-Based Diagnosis for more details.

Pre-test probability of disease

P(D+) =

Likelihood ratio of test result r

LR(r) =

Or, if you don't know the likelihood ratio...

Pre-Test Odds = P/(1 - P) =
Post-Test Odds = Pre-Test Odds * LR(r) =
Post-Test Probability = Post-Test Odds/(1 + Post-Test Odds) =

If you don't know the Likelihood Ratio of the test result:

What is the probability of test result r in a patient WITH the disease?

P(r|D+) =

What is the probability of test result r in a patient WITHOUT the disease?

P(r|D-) =

Calculated Likelihood Ratio: P(r|D+) / P(r|D-)

LR(r) =