The post test probability calculator works out post-test probability, post-test odds and positive likelihood ratio (lr+) from likelihood ratio (lr), false negative, false positive, true negative and true positive. Enter what you have and the relationship is applied straight away, so you get the answer without working through the algebra yourself.
What Is the Post Test Probability Calculator?
The post test probability calculator turns a set of measurements into post-test probability, post-test odds and positive likelihood ratio (lr+), with the uncertainty made explicit rather than left implied.
Statistics is easy to quote badly. A figure without its spread tells the reader far less than they assume it does, and this calculator puts both together.
How the Post Test Probability Is Calculated
The calculation uses the standard relationship for this kind of estimate, applied to the figures you supply. The intermediate steps are exposed so you can see how much of the answer comes from each one.
Sample size matters more than most people expect. Doubling the number of observations improves the precision by a factor of about one and a half, not two.
What the Result Does and Does Not Tell You
A result describes the data you gave it. It does not tell you whether the sample was representative, whether the method was appropriate, or whether there is a confound in the way the figures were collected.
Report the result together with its uncertainty. A number on its own is much easier to misread than a number with a range attached.
A Worked Example
Working through an example, with likelihood ratio (lr) set to 10, false negative set to 5, false positive set to 2, the calculator returns false negative of 5, and false positive of 2, and specificity of 0.98039. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out post-test probability?
Enter likelihood ratio (lr), false negative, false positive, true negative and true positive. The calculator then works through the relationship and reports post-test probability, post-test odds and positive likelihood ratio (lr+).
What does the post test probability actually calculate?
It takes likelihood ratio (lr), false negative, false positive, true negative and true positive and converts them into post-test probability, post-test odds and positive likelihood ratio (lr+) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter likelihood ratio (lr)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.
Do I have to fill in every field?
Only the fields a particular result depends on. If you leave one blank, the results that need it simply show a note instead of a number, and everything else still works.