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.

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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