**Likelihood ratio slide rule (October 24, 2002) **Category:
Diagnostic testing

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The use of likelihood ratios requires a bit of tedious calculations. I have developed

a simple slide rule that will do likelihood ratio calculations for you.Note: I am developing a special handout (PDF format) that explains the mathematics behind diagnostic testing and which illustrates many of the important points using the likelihood ratio slide rule. I distributed this handout in a talk for the American College of Allergy, Asthma & Immunology on Sunday, November 11, but ran out very quickly.

**Assembly instructions**

Please print out this this PDF file. Cut out

the bottom piece (the sleeve) and the top piece (the insert). Also cut out the two rectangles in the middle of the sleeve.Fold the left and right portions of the sleeve behind and tape them together. Double sided tape works very well for this. Slip the insert into the sleeve. You may need to trim a tiny amount off the left and right sides of the insert to get it to fit well.You want the insert to fit not too snugly and not too loosely inside the sleeve.

**For a more durable slide rule**

If you print this to a regular sheet of paper, the slide rule will be okay but a bit flimsy and easy to bend. For a more durable slide rule,

print out the image on a thick piece of paperortape/glue the image to a thin piece of cardboard. You can alsoprint the image on a full sheet adhesive label(like Avery 5165) and thenattach the label to a thick piece of paper or a thin piece of cardboard.

**How to use the slide rule**

Slide the insert up or down until the

pre-test probability in the left window lines up with the likelihood ratio. Read thepost-test probability in the right window.

This work is licensed under a Creative Commons Attribution 3.0 United States License. It was written by Steve Simon.