Difference between revisions of "Specificity And Sensitivity"

Revision as of 19:23, 1 August 2016

Specificity and Sensitivity

Medical journals often report the Specificity and Sensitivity of tests for things like HIV or the Zika virus. These measures describe the rates of Type I and Type II errors.

• Specificity, or sPecificity, concerns the rate of false Positives.

• Sensitivity or seNsitivity concerns the rate of false Negatives.

This is where it gets confusing...

• A false positive result means the truth is negative. So the test might have been a true negative instead of being a false positive, those events are disjoint, and if the truth is negative, the result can't be anything else.

• If the truth is negative, PROBABILITY(False Positive) + PROBABILITY(True Negative) = 1

• Knowing one probability you can find the other

• The Specificity is the Probability of a True Negative, assuming the truth is Negative.

Similarly...

• If the truth is positive, PROBABILITY(False Negative) + PROBABILITY(True Positive) = 1.

• The Sensitivity is the Probability of a True Positive, assuming the truth is Positive.