Unformatted text preview: Sn & Sp refers to the validity of the test • However, on the farm or in the clinic, a diﬀerent ques2on may be of more interest: – If the test result is posiUve/negaUve, what is the probability that this paUent does have / doesn’t have the disease? Relevance of a Test Result (PredicUve Values) PosiUve PredicUve Value (PPV) NegaUve PredicUve Value (NPV) • The probability that an individual with a posi2ve test result has the disease • The probability that an individual with a nega2ve test result doesn’t have the disease PPV = True Posi2ves___ (All +ve tests: TP & FP) PPV = a / (a + b) NPV = True Nega2ves___ (All  ve tests: TN & FN) NPV = d / (c + d) Gold Standard Test +ve  ve +ve a b a + b  ve c d c + d Recall: Chlamydia in Koalas i CLICKERS • What would be the PPV? +ve Test Result – A) 40% – B) 56% – C) 62% – D) 75% – E) Condoms Chlamydia “Gold Standard”  ve +ve 200 125 325  ve 300 375 675 500 500 1000 PPV = a / (a+b) = 200/325 PPV = 62% Recall: Varsity Player Injuries i CLICKERS • What would be the NPV? +ve Test Result – A) 75% – B) 90% – C) 92% – D) 94% – E) 98% Injury “Gold Standard”  ve +ve 90 30 120  ve 10 470 480 100 500 600 NPV = d / (c+d) = 470/480 NPV = 98% Recall: Varsity Player Injuries Prev = 2% Sn = 90% Sp = 94%  ve +ve 90 30 120  ve 10 470 480 100 Test +ve 500 600 Test Prev = 17% Sn = 90% Sp = 94% Injury True Status Injury True Status +ve  ve +ve 9 35 44  ve 3 553 556 12 588 600 • What if the true prev...
View
Full Document
 Fall '13

Click to edit the document details