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Help with questions on specificity and sensitivity

riccoricco Registered User new member
edited November 2009 in Help / Advice Forum
(I realize this is a request for homework help, but I am not asking for you to do my homework but rather for help understanding the concept).

Hello, I have an exam tomorrow in my Psychology class and am struggling with the concept of sensitivity and specificity. I understand that sensitivity is the ratio of true positives to (true positives + false negatives), and specificity is the ratio of true negatives to (true negatives + false positives), as well as the concepts of positive predictive value and negative predictive value.

Unfortunately, the questions on our test will be like the following (from a sample exam) :

Assume that a clinician is 70% correct in being able to predict which former felons will commit new crimes after being paroled. (As a result, he predicts 38% of felons will commit new crimes.) Also assume that the base rate for committing new crimes among a group of 1000 felons is 20%. What is the specificity of this clinician’s predictions?

How do I determine this (and similar questions)? I tried to a draw a chart for it but am lost at which numbers I need to put where.

ricco on
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