AP Stat Chapter 2

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The report of a sample survey of 1,014 adults says, "With 95% confidence, the true population proportion of all Americans that expect to spend more money on gifts this year than last year is between 0.09 and 0.15." What does the phrase "95% confidence" mean?

• 95% of all Americans will spend between 9% and 15% more than what they spent last year.

• 9% to 15% of all Americans will spend 95% of what they spent last year.

• There is a 95% chance that the percent who expect to spend more is between 9% and 15%.

• The method used to get the stated interval, when used over and over, produces intervals which include the true population proportion about 95% of the time.

• The population parameter will be between 0.09 and 0.15 in 95% of all confidence intervals computed in this same manner.

A 95% confidence interval for p, the true proportion of all shoppers at a large grocery store who purchase 2% milk, was found to be (0.436, 0.582). Given that all conditions were met, which of the following is the most appropriate conclusion?

• Based upon these data, the majority of shoppers purchase 2% milk.

• Based upon these data, at least 43.6% of shoppers purchase 2% milk.

• Based upon these data, at least 58.2% of shoppers purchase 2% milk.

• Based upon these data, less than 50% of shoppers purchase 2% milk.

• Based upon these data, 95% of shoppers purchase 2% milk.

An SRS of 100 flights by Speedy Airlines showed that 75 were on time. An SRS of 100 flights by Happy Airlines showed that 80 were on time. Let ps be the proportion of on-time flights for all Speedy Airline flights, and let ps be the proportion of all on-time flights for all Happy Airlines flights. A 95% confidence interval for the difference ps - pH is -0.166 to 0.066. Based on this confidence interval, which conclusion is best?

• Because 0 is in the interval, there is convincing evidence of a difference in the population proportions.

• Because 0 is in the interval, there is not convincing evidence of a difference in the population proportions.

• Because the point estimate (-0.05) is negative, there is convincing evidence of a difference in the population proportions.

• Because the point estimate (-0.05) is negative, there is not convincing evidence of a difference in the population proportions.

• Because most of the interval is negative, there is convincing evidence that a greater proportion of Speedy Airlines flights are on time.