To determine whether or not they have a certain disease, 140 people are to have

Question

To determine whether or not they have a certain disease, 140 people are to have their blood tested. However, rather than testing each individual separately, it has been decided first to group the people in groups of 10. The blood samples of the 10 people in each group will be pooled and analyzed together. If the test is negative. one test will suffice for the 10 people (we are assuming that the pooled test will be positive if and only if at least one person in the pool has the disease); whereas, if the test is positive each of the 10 people will also be individually tested and, in all, 11 tests will be made on this group.
Assume the probability that a person has the disease is 0.07 for all people, independently of each other.

Compute the expected number of tests necessary for each group.

Explanation / Answer

For a group of 10 people the probability that at least one person has the disease equals 1 - the probability that none of the 10 have the disease. For each person, the probability of not having the disease is (1 - 0.07) = 0.93

E(x) = 11 - 10 * 0.93^10

E(x) = 6.16

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