Question

An environmental agency worries that many cars may be violating clean air emissions standards. The agency hopes to check a sample of vehicles in order to estimate that percentage with a margin of error of 5​% and 95​% confidence. To gauge the size of the​ problem, the agency first picks 50 cars and finds 12 with faulty emissions systems. How many should be sampled for a full​ investigation?

Answer 1

197

Explanation:

Sample proportion is; p^ = 12/50 = 0.24

Margin of error of 5% is given by the formula;

ME = (z_0.05) √[p^(1 - p^)/n]

Let's make n(number of sample) the subject.

n = ((z_0.05)/ME)²((p^)(1 - p^))

From tables, the z-score of 0.05 is 1.645

Thus;

n = ((1.645)/0.05)²(0.24(1 - 0.24))

n ≈ 197