Based on a random sample of 50, a 95% confidence interval for the population proportion was computed. Holding everything else constant, which of the following will reduce the length of the confidence interval by half? (CHECK ALL THAT APPLY): A. Quadruple the sample size. B. Change the confidence level to 68%. C. Double the sample size. D. Change the confidence level to 99.7%. E. Decrease the sample proportion by half.

The length of the confidence interval is the margin of error, which is the ratio of the standard deviation and the square root of sample size. Hence, to reduce the length of confidence interval by half, Quadruple the sample size.

Recall :

Margin of Error = σ/√n

Evaluating an hypothetical scenario :

Let standard deviation, σ = 2

Sample size = 50

Margin of Error = 2/√50 = 0.554

Using Quadruple of the sample size : (50 × 4) = 200 samples

Margin of Error = 2/√200 = 0.277

(0.227 ÷ 0.554) = 0.5

Therefore, increasing the sample size, reduces the margin of error. Hence, using quadruple the sample size, will reduce the margin of error by half.

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