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S05 Screening & Diagnostic Tests (FILLED IN)

# How did they change ppv 944 205 npv 553556

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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...
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