 Solution

ID: past-exam-of-the-options-i-course-of-the-university-of-oxford/2008/o/o1/ii/solution

Under the null, the common fitted CC frequency is its pooled proportion

133/3874

. Multiplying by the fixed number of cases gives

E_{CC, case} = 1939 \frac{83 + 50}{1939 + 1935} = 66.5687 \dots \approx 66.6 .

This estimates the common type frequency from both groups, rather than using the case frequency alone.

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