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The likelihood ratio is the ratio of the probability of a given test result among people with a specific target condition to the probability of that same test result among people without the target condition.
When the test result is positive, the likelihood ratio is known as a positive likelihood ratio. When the test result is negative, the likelihood ratio is known as a negative likelihood ratio.
Likelihood ratios are used for assessing the value of using a diagnostic test, and can be used to calculate the probability of someone having a target condition based on the person’s pre-test probability and the test result.
A likelihood ratio of 1 means that a test result adds no additional diagnostic information to the pre-test probability of having the condition, so that the test result does not alter the likelihood of having the condition. The larger the positive likelihood ratio is (above 1) and the smaller the negative likelihood ratio is (below 1), the more informative the diagnostic test is.
For example, a diagnostic test called a large core needle biopsy, when used to diagnose breast cancer, has a positive likelihood ratio of 16.2 and a negative likelihood ratio of 0.03.
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