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.