How to Evaluate a 2 by 2 Table
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Evaluating a 2 by 2 table is simple if you are methodical in your approach.
|
Disease
present |
Disease
absent |
|
Test
positive |
TP |
FP |
Total
positive |
Test
negative |
FN |
TN |
Total
negative |
|
Total with
disease |
Total with-
out disease |
Grand
total |
-
Choose an arbitrary (large) number of hypothetical patients and write it
in the Grand total cell.
-
Multiply the Grand total by the Pretest probability to get
the Total with disease.
-
Compute the Total without disease by subtraction.
-
Multiply the Total with disease by the Sensitivity to get
the number of True positives.
-
Multiply the Total without disease by the Specificity to
get the number of True Negatives.
-
Compute the number of False positives and False negatives
by subtraction.
-
Compute the Total positive tests and Total negative tests
by addition across the rows.
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Predictive value of a positive test is True positives divided
by Total positive tests.
-
Predictive value of a negative test is True negatives divided
by Total negative tests.
Click here for practice problems on evaluating 2 by
2 tables.
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