Two radiologists were asked to classify 150 CT scans as abnormal or normal indep
ID: 3405501 • Letter: T
Question
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%.
Radiologist 2
Radiologist 1
Abnormal
Normal
Total
Abnormal
70
20
90
Normal
15
45
60
Total
85
65
150
Explanation / Answer
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
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