D.20 Answers: More about formings CIs

Answers to exercises in Sect. 21.5.

Answer to Exercise 21.1: The conclusion states that the interval is one in which they are reasonably sure (i.e., 95% sure) that the sample proportion will lie. But the researcher knows exactly what the sample proportion is: it is \(\hat{p} = 0.314\).

CIs give intervals in which we are reasonably certain that the population value is within, because the population proportion is unknown.

(In addition, the CI is a 68% anyway, not a 95% CI as claimed.)
Answer to Exercise 21.2: The CI is not about individual trees; it is about a sample statistic. Presumably, it should read something like ‘This CI means that between 22.3% and 40.5% of trees are infected with apple scab.’