Q: A psychiatrist devised a short screening test for depression. An independent blind comparison was made with a gold standard for diagnosis of depression among 200 psychiatric outpatients. Among the 50 outpatients found to be depressed according to the gold standard, 35 patients were positive for the test. Among 150 patients found not to be depressed according to the gold standard, 30 patients were found to be positive for the test.

a. The sensitivity was 80%
b. The specificity was 80%
c. The positive predictive value was 80%
d. The negative predictive value was 80%

 

 

 

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Ans: b.

 

Step 1: Make a 2x2 table in this format:

	Gold Std +	Gold Std -	Total
New Test +
New Test  -
Total

 

Step 2: Start filling this table based on data given:

	Gold Std +	Gold Std -	Total
New Test +	35	30
New Test  -
Total	50	150	200

 

Step 3: Complete remaining table :

	Gold Std +	Gold Std -	Total
New Test +	35	30	65
New Test  -	15	120	135
Total	50	150	200

Step 4: Use definitions to calculate indices:

• 
  

  Sensitivity = True Positive / Total Diseased           =  35/50 =  0.7 (or 70%)

• 
  

  Specificity = True Negative / Total Not diseased    = 120/150 = 0.8  (or 80%)

• 
  

  PPV = True Positive / All Positives                        = 35/65 = 0.54 (or 54%)

• 
  

  NPV = True Negative / All Negatives                     = 120 / 135 = 0.89 (or 89%)