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
 

  1. Choose an arbitrary (large) number of hypothetical patients and write it in the Grand total cell.
  2. Multiply the Grand total by the Pretest probability to get the Total with disease.
  3. Compute the Total without disease by subtraction.
  4. Multiply the Total with disease by the Sensitivity to get the number of True positives.
  5. Multiply the Total without disease by the Specificity to get the number of True Negatives.
  6. Compute the number of False positives and False negatives by subtraction.
  7. Compute the Total positive tests and Total negative tests by addition across the rows.
  8. Predictive value of a positive test is True positives divided by Total positive tests.
  9. 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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