AP Statistics : How to find confidence intervals

Study concepts, example questions & explanations for AP Statistics

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Example Questions

Example Question #1 : How To Find Confidence Intervals

Which of the following statements are correct about confidence intervals? 

Possible Answers:

The width of a confidence interval decreases as the sample size increases and increases as the confidence level decreases.

The width of a confidence interval does not change as the sample size increases and increases as the confidence level increases.

The width of a confidence interval increases as the sample size increases and increases as the confidence level decreases.

The width of a confidence interval increases as the sample size increases and increases as the confidence level increases.

The width of a confidence interval decreases as the sample size increases and increases as the confidence level increases.

Correct answer:

The width of a confidence interval decreases as the sample size increases and increases as the confidence level increases.

Explanation:

Larger samples give narrower intervals. We are able to estimate a population proportion more precisely with a larger sample size. 

As the confidence level increases the width of the confidence interval also increases. A larger confidence level increases the chance that the correct value will be found in the confidence interval. This means that the interval is larger. 

Example Question #2 : How To Find Confidence Intervals

You are asked to create a  confidence interval with a margin of error no larger than  while sampling from a normally distributed population with a standard deviation of . What is the minimum required sample size?

Possible Answers:

Correct answer:

Explanation:

Keep in mind that the margin of error for a confidence interval based on a normal population is equal to , where  is the -score corresponding to the desired confidence level.

From the problem, we can tell that  and . We can then solve for  algebraically:

The minimum sample size is  rounded up, which is . If you are unsure on problems like these, you can check the margin of error for your answer rounded down and then rounded up (in this case, for  and .)

Example Question #11 : Confidence Intervals

Jim calculated a  confidence interval for the mean height in inches of boys in his high school. He is not sure how to interpret this interval. Which of the following explains the meaning of  confidence.

Possible Answers:

More information is needed.

In the long run,  of all confidence intervals calculated from the same population will contain the true mean height.

 of boys' heights fall with the interval Jim calculated.

 of all possible sample means fall within Jim's interval.

There is a  chance that Jim's interval contains the true mean height.

Correct answer:

In the long run,  of all confidence intervals calculated from the same population will contain the true mean height.

Explanation:

95% confidence means that the methods Jim uses to calculate his confidence interval give him correct results 95% of the time. It does not mean that there is a 95% chance that the true mean will be inside the interval. It also does not mean that 95% of all heights or possible sample means fall within the interval.

Example Question #3 : How To Find Confidence Intervals

The number of hamburgers served by McGregors per day is normally distributed and has a mean of  hamburgers and a standard deviation of  Find the range of customers served on the middle  percent of days. 

Possible Answers:

Correct answer:

Explanation:

First, find the first quartile of the distribution.

Then, find the third quartile of the distribution. 

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