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Home : Top directory : / Journal Review / Statistics / Sensitivity and Specificity

Sensitivity and Specificity

Author:Lengyel J.
on 28 Nov, 2007

Last edited by: Lengyel J. on 28 Nov, 2007
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See Wkipedia for a worked example http://en.wikipedia.org/wiki/Sensitivity_%28tests%29
Relationships among terms
    Condition
(as determined by "Gold standard")
    True False  
Test
outcome		 Positive True Positive False Positive
(Type I error, P-value)		 → Positive predictive value
Negative False Negative
(Type II error)		 True Negative → Negative predictive value
    ↓
Sensitivity		 ↓
Specificity
A worked example
the Fecal occult blood (FOB) screen test is used in 203 people to look for bowel cancer:
    Patients with bowel cancer
(as confirmed on endoscopy)
    True False  ?
FOB
test		 Positive TP = 2 FP = 18 = TP / (TP + FP)
= 2 / (2 + 18)
= 2 / 20 ≡ 10%
Negative FN = 1 TN = 182 = TN / (TN + FN)
182 / (1 + 182)
= 182 / 183 ≡ 99.5%
    ↓
= TP / (TP + FN)
= 2 / (2 + 1)
= 2 / 3 ≡ 66.67%		 ↓
= TN / (FP + TN)
= 182 / (18 + 182)
= 182 / 200 ≡ 91%

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