Worked Example
Suppose the fecal occult blood (FOB) screen test is used in 2030 people to look for bowel cancer:
| Patients with bowel cancer (as confirmed on endoscopy) |
||||
| Condition Positive | Condition Negative | |||
| Fecal Occult Blood Screen Test Outcome |
Test Outcome Positive |
True Positive (TP) = 20 |
False Positive (FP) = 180 |
Positive predictive value
= TP / (TP + FP) = 20 / (20 + 180) = 10% |
| Test Outcome Negative |
False Negative (FN) = 10 |
True Negative (TN) = 1820 |
Negative predictive value
= TN / (FN + TN) = 1820 / (10 + 1820) ≈ 99.5% |
|
| Sensitivity
= TP / (TP + FN) = 20 / (20 + 10) ≈ 67% |
Specificity
= TN / (FP + TN) = 1820 / (180 + 1820) = 91% |
|||
The small positive predictive value (PPV = 10%) indicates that many of the positive results from this testing procedure are false positives. Thus it will be necessary to follow up any positive result with a more reliable test to obtain a more accurate assessment as to whether cancer is present. Nevertheless, such a test may be useful if it is inexpensive and convenient. The strength of the FOB screen test is instead in its negative predictive value - which, if negative for an individual, gives us a high confidence that its negative result is true.
Read more about this topic: Positive Predictive Value
Famous quotes containing the word worked:
“The conviction that the best way to prepare children for a harsh, rapidly changing world is to introduce formal instruction at an early age is wrong. There is simply no evidence to support it, and considerable evidence against it. Starting children early academically has not worked in the past and is not working now.”
—David Elkind (20th century)
“It doesn’t seem so much to climb a mountain
You’ve worked around the foot of all your life.”
—Robert Frost (1874–1963)