Two radiologists were asked to classify 150 CT scans as abno

Two radiologists were asked to classify 150 CT scans as abnormal or normal independently. The comparison of their classification is shown below:

                  Radiologist 2

Radiologist 1

Abnormal

Normal

Total

Abnormal

70

20

90

Normal

15

45

60

Total

85

65

150

Calculate the overall percent agreement between the two radiologists.

What is the percent agreement between the two radiologists, excluding the CT scans that both radiologists classified as normal?

Calculate the value of kappa statistic? Assume that the percent agreement expected by chance alone is equal to 51%.

Solution

Total CT scans = 150
Number of cases in which both radiologist classified it as Abnormal = 70
Number of cases in which both radiologist classified it as Normal = 45
Hence percentage agreement = (70 + 45)/150 = 76.66%

Now, excluding the cases that both radiologist classified normal = 70/(150-45) = 66.66%

Kappa statistic (k) = {P(a) - P(e)}/(1 - P(e) ; where P(a) : total proportion of cases in which there is an agreement
   P(e) : proportion of agreement expected by chance alone
Hence P(a) = (70+45)/150 = 0.767
  and P(e) = 0.51 (given)
Therefore k = (0.767 - 0.51)/(1 - 0.51) = 0.5244