All::Clinical Science::Statistics::Pre- and post- test odds and probability
Intro
What is meant by Pre-test probability?
the probability of the presence of a condition in a population
What is meant by post-test probability?
The proportion of patients with that particular test result who have the target disorder
What is meant by\ Pre-test odds?
The odds that the patient has the target disorder before the test is carried out
What is meant by post-test odds?
The odds that the patient has the target disorder after the test is carried out
How is Post-test probability calculated? (from post test odds)
Post-test probability = post test odds / (1 + post-test odds)
How is Pre-test odds calculated? (from pre-test probability)
Pre-test odds = pre-test probability / (1 - pre-test probability)
How is Post-test odds calculated? (from likelihood ratio)
Post-test odds = pre-test odds x likelihood ratio
where the likelihood ratio for a positive test result = sensitivity / (1 - specificity)
How is likelihood ratio for a positive test result calculated?
sensitivity / (1 - specificity)
What is the difference between odds and probability?
- Odds is a comparison between the chance an outcome happens vs the change it doesn't happen
- Probability is a comparison between the chance an outcome happens vs all outcomes