This specificity calculator takes the figures you already have and applies the relationship behind specificity calculations for you, giving ppv (positive predictive value), npv (negative predictive value) and positive likelihood ratio from false negatives, false positives, prevalence, true negatives and true positives.
What Is the Specificity Calculator?
The specificity calculator turns a set of measurements into ppv (positive predictive value), npv (negative predictive value) and positive likelihood ratio, 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 Specificity 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 false negatives set to 10, false positives set to 5, prevalence set to 2, the calculator returns sensitivity of 0.5, and specificity of 0.95238, and positive likelihood ratio of 10.5. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out ppv (positive predictive value)?
Enter false negatives, false positives, prevalence, true negatives and true positives. The calculator then works through the relationship and reports ppv (positive predictive value), npv (negative predictive value) and positive likelihood ratio.
What does the specificity actually calculate?
It takes false negatives, false positives, prevalence, true negatives and true positives and converts them into ppv (positive predictive value), npv (negative predictive value) and positive likelihood ratio 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 false negatives?
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.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.
Does anything you type get stored?
No. The whole calculation runs inside your browser, and nothing you enter is uploaded, logged or sent anywhere.