Difference of Two Population Proportions (Test)

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AP Statistics › Difference of Two Population Proportions (Test)

Questions 1 - 10
1

A public health researcher wants to know whether the proportion of adults who received a flu shot differs between City 1 and City 2. In a random sample of 250 adults from City 1, 145 reported getting a flu shot. In a random sample of 220 adults from City 2, 110 reported getting a flu shot. A two-proportion $z$ test was conducted for $H_0: p_1=p_2$ versus $H_a: p_1\ne p_2$, yielding a p-value of 0.018. Using $\alpha=0.05$, what conclusion is appropriate?

Because the p-value is 0.018, living in City 1 causes people to be more likely to get a flu shot.

Because the p-value is 0.018, we should fail to reject $H_0$ because 0.018 is greater than 0.05.

Because the p-value is 0.018, there is sufficient evidence that the population proportions of adults who received a flu shot are different in City 1 and City 2.

Because the p-value is 0.018, we can conclude that the samples have different proportions, so the cities must differ.

Because the p-value is 0.018, there is sufficient evidence that City 2 has a higher population proportion of adults who received a flu shot than City 1.

Explanation

This question evaluates understanding of a two-sided two-proportion z-test for differences in flu shot proportions between two cities. The p-value of 0.018 is less than alpha=0.05, so we reject the null hypothesis, concluding there is sufficient evidence of a difference in the population proportions. Choice B is a distractor because it specifies a direction (City 2 higher), but the two-sided alternative only supports a difference without indicating which is larger. Conclusions in two-proportion tests should emphasize inference to populations, not just observed sample differences, and avoid assuming causation from observational data. For two-sided tests, rejection means evidence of inequality, but further analysis like confidence intervals would be needed to determine direction. This highlights the importance of matching the conclusion to the test type and significance level.

2

A fitness center compared the proportion of members who renew their membership after one year under two pricing plans. In a random sample of 180 members on Plan X, 117 renewed; in a random sample of 200 members on Plan Y, 110 renewed. A two-proportion $z$ test was conducted for $H_0: p_X=p_Y$ versus $H_a: p_X>p_Y$, and the p-value was $0.012$. Using $\alpha=0.05$, what conclusion is appropriate?

Reject $H_0$; there is sufficient evidence that the population renewal proportion is higher for Plan Y than for Plan X.

Reject $H_0$; Plan X causes members to renew at a higher rate than Plan Y.

Reject $H_0$; there is sufficient evidence that the population renewal proportion is higher for Plan X than for Plan Y.

Reject $H_0$; there is sufficient evidence that the two samples have different renewal proportions.

Fail to reject $H_0$; there is not sufficient evidence that the population renewal proportion for Plan X is higher than for Plan Y.

Explanation

This problem involves a one-tailed test with H_a: p_X > p_Y, testing if Plan X has a higher renewal proportion. With p-value = 0.012 < α = 0.05, we reject H₀. The correct conclusion is that there's sufficient evidence that the population renewal proportion is higher for Plan X than for Plan Y (choice A). Choice B reverses the direction. Choice C would be correct if we failed to reject H₀. Choice D incorrectly implies causation from an observational comparison. Choice E refers to sample proportions and doesn't specify direction. In one-tailed tests, the conclusion must match the alternative hypothesis direction, and we conclude about population parameters. The small p-value provides strong evidence supporting the claim that Plan X has a higher population renewal rate.

3

A political scientist compared the proportion of registered voters who approve of a policy in two independent random samples. Among 500 voters in Region North, 255 approved; among 450 voters in Region South, 207 approved. A two-proportion $z$ test at $\alpha=0.05$ tested $H_0: p_N=p_S$ versus $H_a: p_N\ne p_S$, where $p_N$ and $p_S$ are the true approval proportions in the two regions. The p-value was 0.18. What conclusion is appropriate?

Because the p-value is 0.18, there is convincing evidence that approval is different in the two regions.

Because the p-value is 0.18, we conclude that 255 approvals in North and 207 approvals in South means North approves more in the population.

Because the p-value is 0.18, there is convincing evidence that Region South has a higher true approval proportion than Region North.

Because the p-value is 0.18, the policy causes voters in Region North to approve more than voters in Region South.

Because the p-value is 0.18, there is not convincing evidence of a difference in the true approval proportions between Region North and Region South.

Explanation

This question tests understanding of two-proportion z-test conclusions for voter approval rates. With a p-value of 0.18 and α = 0.05, we fail to reject the null hypothesis since 0.18 > 0.05. This means there is not convincing evidence of a difference in the true approval proportions between the regions. Choice B correctly states this conclusion. Choice A incorrectly claims evidence of difference when the large p-value indicates otherwise. When we fail to reject H₀, we cannot conclude the proportions are different, regardless of the observed sample proportions. The two-sided test (≠) examines whether any difference exists, and the high p-value suggests the observed difference could reasonably occur by chance alone.

4

A school district compared the proportion of students who passed an end-of-course exam after two different review programs. In a random sample of 200 students using Program A, 128 passed; in a separate random sample of 180 students using Program B, 99 passed. A two-proportion $z$ test was performed for $H_0: p_A=p_B$ versus $H_a: p_A\ne p_B$, and the p-value was $0.006$. Using $\alpha=0.05$, what conclusion is appropriate?

Fail to reject $H_0$; there is not sufficient evidence that the population pass rates differ between Program A and Program B.

Reject $H_0$; there is sufficient evidence that Program A causes a higher pass rate than Program B.

Reject $H_0$; there is sufficient evidence that the population pass rate for Program A differs from the population pass rate for Program B.

Reject $H_0$; there is sufficient evidence that the sample pass rates differ between the two samples.

Reject $H_0$; there is sufficient evidence that the population pass rate for Program B differs from the population pass rate for Program A, because $p_B>p_A$.

Explanation

This question tests understanding of hypothesis testing for the difference of two population proportions. With a p-value of 0.006, which is less than α = 0.05, we reject the null hypothesis. The correct interpretation is that there is sufficient evidence that the population pass rates differ between Program A and Program B (choice B). Choice C incorrectly claims causation and specifies direction when we used a two-tailed test. Choice D incorrectly refers to sample proportions rather than population proportions. Choice E incorrectly uses sample notation (p_B > p_A) when discussing population parameters. When rejecting H₀ in a two-proportion test, we conclude there's evidence of a difference in population proportions, not sample proportions or causal relationships.

5

An environmental group compared the proportion of households that recycle weekly in two neighboring towns. In a random sample of 160 households in Town A, 92 reported recycling weekly; in a random sample of 150 households in Town B, 78 reported recycling weekly. A two-proportion $z$ test was conducted for $H_0: p_A=p_B$ versus $H_a: p_A\ne p_B$, and the p-value was $0.29$. Using $\alpha=0.05$, what conclusion is appropriate?

Reject $H_0$; there is sufficient evidence that the population weekly recycling proportions differ between Town A and Town B.

Reject $H_0$; there is sufficient evidence that Town B’s population weekly recycling proportion is greater than Town A’s.

Fail to reject $H_0$; there is not sufficient evidence that the population weekly recycling proportions differ between Town A and Town B.

Fail to reject $H_0$; there is not sufficient evidence that the sample weekly recycling proportions differ between the two samples.

Fail to reject $H_0$; therefore, Town A and Town B have exactly the same population weekly recycling proportion.

Explanation

This problem involves a two-tailed test for comparing recycling proportions between towns. With p-value = 0.29 > α = 0.05, we fail to reject the null hypothesis. The correct conclusion is that there's not sufficient evidence that the population weekly recycling proportions differ between Town A and Town B (choice B). Choice C incorrectly claims the proportions are exactly equal - failing to reject H₀ doesn't prove equality. Choice D incorrectly refers to sample proportions. Choice E would require rejecting H₀ and having a one-tailed test. When we fail to reject H₀, we cannot conclude the null hypothesis is true; we can only say there's insufficient evidence to support the alternative hypothesis. This distinction is crucial in hypothesis testing.

6

A school district compares two methods for encouraging homework completion. In a random sample of 180 students using Method A, 126 completed homework every day last week. In a separate random sample of 200 students using Method B, 120 completed homework every day last week. A two-proportion $z$ test was performed for $H_0: p_A=p_B$ versus $H_a: p_A>p_B$, producing a p-value of 0.002. At the 5% significance level, what conclusion is appropriate?

Because the p-value is 0.002, we can conclude that 126/180 is greater than 120/200 in these samples.

Because the p-value is 0.002, we can conclude that Method A caused more of the sampled students to complete homework daily.

Because the p-value is 0.002, there is convincing evidence that Method B has a higher population proportion of daily homework completion than Method A.

Because the p-value is 0.002, there is not sufficient evidence of a difference in the population proportions between the two methods.

Because the p-value is 0.002, there is convincing evidence that the population proportion of students who complete homework daily is higher with Method A than with Method B.

Explanation

This question tests the skill of interpreting a two-proportion z-test for comparing population proportions of homework completion between two methods. With a p-value of 0.002 less than the typical alpha of 0.05, we reject the null hypothesis, providing convincing evidence that the population proportion is higher for Method A than Method B, as stated in the alternative hypothesis. A common distractor is choice A, which reverses the direction by claiming Method B is higher, but the sample data and alternative hypothesis support Method A having the higher proportion. In drawing conclusions from a two-proportion z-test, we focus on population parameters rather than sample statistics alone and avoid causal claims unless the study design supports it, such as through randomization. Here, the test allows us to infer a difference in populations but not that one method causes better completion rates. Remember, the p-value indicates the strength of evidence against the null, and rejecting it aligns with the direction specified in the alternative hypothesis.

7

A nutritionist investigates whether a larger proportion of teens drink soda daily in Region A than in Region B. In a random sample of 210 teens from Region A, 84 reported drinking soda daily. In a random sample of 230 teens from Region B, 78 reported drinking soda daily. A two-proportion $z$ test for $H_0: p_A=p_B$ versus $H_a: p_A>p_B$ produced a p-value of 0.084. Using $\alpha=0.10$, what conclusion is appropriate?

Because the p-value is 0.084, we can conclude that the two sample proportions are different, so the population proportions must be different.

Because the p-value is 0.084, Region A causes teens to drink soda daily more often than Region B.

Reject $H_0$; there is sufficient evidence that the population proportion is higher in Region B than in Region A.

Fail to reject $H_0$; there is not sufficient evidence that the population proportion is higher in Region A than in Region B.

Reject $H_0$; there is sufficient evidence that the population proportion of teens who drink soda daily is higher in Region A than in Region B.

Explanation

This question assesses a one-sided two-proportion z-test for daily soda drinking proportions between regions. The p-value of 0.084 is less than alpha=0.10, so we reject the null, finding sufficient evidence that Region A has a higher population proportion than Region B. Choice B distracts by suggesting failure to reject, but 0.084 falls below the given 0.10 threshold. In conclusions, focus on populations and note that different alphas can change decisions—here, it would fail at 0.05 but rejects at 0.10. Avoid causation, as regions likely involve observational data. This shows the role of significance level in decision-making.

8

A sports scientist compares the proportion of athletes who report reduced knee pain after 6 weeks using two training programs. In independent random samples, 58 of 100 athletes using Program P reported reduced pain and 49 of 100 using Program Q reported reduced pain. A two-proportion $z$ test for $H_0: p_P-p_Q=0$ versus $H_a: p_P-p_Q>0$ resulted in a p-value of $0.11$. At $\alpha=0.05$, what conclusion is appropriate?

Reject $H_0$; there is convincing evidence that the population proportion reporting reduced pain is higher for Program P than for Program Q.

Fail to reject $H_0$; therefore 58/100 and 49/100 are not meaningfully different in these samples.

Fail to reject $H_0$; there is not convincing evidence that Program P has a higher population reduced-pain proportion than Program Q.

Because the p-value is 0.11, Program P and Program Q work equally well for all athletes.

Reject $H_0$; there is convincing evidence that the population proportion reporting reduced pain is higher for Program Q than for Program P.

Explanation

This question tests whether Program P has a higher population proportion of athletes reporting reduced pain than Program Q. The p-value (0.11) exceeds α = 0.05, so we fail to reject H₀. This means there is not convincing evidence that Program P has a higher population reduced-pain proportion than Program Q, despite the sample showing 58/100 versus 49/100. Choice D incorrectly interprets the p-value as proving the programs work equally well. Choice E misunderstands the purpose of hypothesis testing, which is to make inferences about populations. When we fail to reject H₀, we're saying the sample difference could reasonably occur by chance if the population proportions were equal.

9

A teacher compares two study methods to see if Method A leads to a higher proportion of students earning an A on the next quiz. Students were randomly assigned to methods. In Method A, 33 of 80 earned an A; in Method B, 25 of 80 earned an A. A two-proportion $z$ test for $H_0: p_A-p_B=0$ versus $H_a: p_A-p_B>0$ gave a p-value of $0.09$. At $\alpha=0.05$, what conclusion is appropriate?

Fail to reject $H_0$; therefore the two population proportions of A grades are exactly equal.

Fail to reject $H_0$; there is not convincing evidence that Method A has a higher population A-rate than Method B.

Reject $H_0$; there is convincing evidence that Method B results in a higher population proportion of A grades than Method A.

Reject $H_0$; there is convincing evidence that Method A results in a higher population proportion of A grades than Method B.

Because students were randomly assigned, the p-value of 0.09 proves Method A causes a higher A-rate.

Explanation

This problem involves testing whether Method A leads to a higher proportion of A grades than Method B. The p-value (0.09) is greater than α = 0.05, so we fail to reject the null hypothesis. This means there is not convincing evidence that Method A has a higher population A-rate than Method B, despite the sample showing 33/80 versus 25/80. Choice D incorrectly claims the p-value proves causation. Choice E incorrectly states that failing to reject H₀ proves the proportions are exactly equal. When the p-value exceeds the significance level, we lack sufficient evidence to support the alternative hypothesis, but this doesn't prove the null hypothesis is true.

10

A conservation group compared the proportion of households that support a new recycling ordinance in two independent random samples. In Town A, 150 of 240 households supported the ordinance; in Town B, 132 of 260 households supported it. A two-proportion $z$ test at $\alpha=0.05$ tested $H_0: p_A=p_B$ versus $H_a: p_A>p_B$, where $p_A$ and $p_B$ are the true support proportions in Town A and Town B. The p-value was 0.004. What conclusion is appropriate?

Because the p-value is 0.004, there is convincing evidence that Town B’s true support proportion is greater than Town A’s.

Because the p-value is 0.004, living in Town A causes households to support the ordinance more than living in Town B.

Because the p-value is 0.004, we conclude that 150 households in Town A will always support the ordinance.

Because the p-value is 0.004, there is convincing evidence that Town A’s true support proportion is greater than Town B’s.

Because the p-value is 0.004, there is not convincing evidence that Town A’s true support proportion is greater than Town B’s.

Explanation

This question involves a one-sided test comparing recycling ordinance support between towns. With a p-value of 0.004 and α = 0.05, we reject the null hypothesis since 0.004 < 0.05. The alternative hypothesis H_a: p_A > p_B specifically tests whether Town A has higher support than Town B. The very small p-value provides convincing evidence for this directional claim. Choice A correctly states this conclusion. Choice B incorrectly claims no evidence when the p-value is well below α. A p-value of 0.004 indicates the observed data would be very unlikely if the null hypothesis were true. In one-sided tests, small p-values provide strong evidence for the specific directional claim in the alternative hypothesis.

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