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AP Statistics/Unit 5: Sampling Distributions
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Probability for Difference in Proportions

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The table below summarizes data from two independent random samples of customers. Which of the following correctly computes the probability that the difference in sample proportions (revamped minus unrevamped) is greater than 0.10?

Restaurant Sample Size (n) Sample Proportion (p̂)
Revamped 110 0.25
Unrevamped 120 0.19
A

Apply an unwarranted continuity correction leading to a probability calculation around 0.5, which misapplies normal approximation techniques.

B

Reverse the difference in the numerator to compute z=0.06−0.100.0547z = \frac{0.06 - 0.10}{0.0547}, which incorrectly yields a negative z-score and an incorrect probability.

C

Assume a negligible standard error, concluding erroneously that the probability is 0.

D

First, compute the standard error as σ=0.25∗0.75110+0.19∗0.81120≈0.0547\sigma = \sqrt{\frac{0.25*0.75}{110} + \frac{0.19*0.81}{120}} \approx 0.0547. Then, find the z-score z=0.10−0.060.0547≈0.731z = \frac{0.10 - 0.06}{0.0547} \approx 0.731, so that P(Z>0.731)≈0.232P(Z > 0.731) \approx 0.232.

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