Using Likelihood Ratios

Using Likelihood Ratios
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Understanding Likelihood Ratios
How much do LR's change disease likelihood?

User's Guide: Diagnostic Tests
Understanding Likelihood Ratios

The likelihood ratio for a test result compares the likelihood of that result in patients with disease to the likelihood of that result in patients without disease:

	D+	D-
T+	a	b	LR+ = (a/a+c)/(b/b+d)
T-	c	d	LR- = (c/a+c)/(d/b+d)

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How much do LRs change disease likelihood?

LRs > 10 or <0.1 cause large changes in likelihood.
LRs 5-10 or 0.1-0.2 cause moderate changes.
LRs 2-5 or 0.2-0.5 cause small changes.
LRs > 2 or >0.5 cause tiny changes.
LRs of 1.0 cause no change at all.

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