Thirteen cases of leukemia are reported in people living in a particular sub-div

Question

Thirteen cases of leukemia are reported in people living in a particular sub-division over a 5-year period. Based on national cancer-incidence rates, for a sub-division of this size, only 4.8 cases would be expected (average # leukemia cases) with a standard deviation of 2.55. Is this number of cases abnormal? Choose the best answer below?

a. Yes, because 13 is almost three times as large as 4.8 (= the average # of leukemia cases). Thus, the # of cases of leukemia in this sub-division is 2 standard deviations away from the mean which makes it abnormal.

b. No, because the number of people with leukemia will naturally vary from sub-division to sub-division. Thus, due to chance we expect to see numbers as large or larger than 13.

c. Yes, because 13 is more than 3 standard deviations away from the mean. Thus, this number of cases of leukemia is outside the range of values we would expect to see just due to chance and so it is abnormal.

d. No, because 13 is only 3.21 standard deviations away from the mean. Thus, this number of cases of leukemia it is inside of the range of values we would expect to see just due to chance and so it is not abnormal.

Explanation / Answer

option C is correct

c. Yes, because 13 is more than 3 standard deviations away from the mean. Thus, this number of cases of leukemia is outside the range of values we would expect to see just due to chance and so it is abnormal.

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