Confidence intervals

Practice sheet

Fresh questions from the whole chapter, to do on paper.

To save a PDF, choose “Save as PDF” in the print dialog.

  1. A 95% confidence interval for a proportion is (39/100, 49/100). Find the margin of error.
  2. A confidence interval for a mean is (41, 55). Find the point estimate.
  3. A 95% interval from a sample of 591 is too wide. Which change makes it narrower?
    1. A larger sample size
    2. A higher confidence level, such as 99%
    3. Nothing: the width is fixed by the population
  4. In a random sample of 400, p̂ = 5/10. Using z* = 1.96, find the margin of error of a 95% interval for p.
  5. In a random sample of 100, p̂ = 3/10. Using z* = 2, find the upper end of the approximate 95% interval for p.
  6. A 95% confidence interval for the proportion of voters who support a plan is (0.48, 0.52). Which interpretation is right?
    1. We are 95% confident that the interval from 0.48 to 0.52 captures the true proportion of supporters
    2. 95% of the sample supports the plan
    3. There is a 95% probability that p is between 0.48 and 0.52
  7. From the same sample of 458, a 99% and a 90% interval are computed. Which is wider?
    1. The 99% interval
    2. The 90% interval
    3. They have the same width
  8. What does the confidence level 90% mean?
    1. In repeated random sampling, about 90% of the intervals built this way capture the parameter
    2. This interval contains the parameter with probability 90%
    3. 90% of the data fall in the interval
  9. A 95% interval (z* = 1.96) has margin of error 0.07. From the same data, find the margin of error of a 99% interval (z* = 2.576).
  10. Using p = 0.5 and z* = 1.96, find the value of n (before rounding up) needed for a margin of error of 0.04.
  11. A survey of 264 people gives a margin of error you want to halve. About how many people are needed?
    1. 1056
    2. 528
    3. About half as many
  12. A margin of error is 0.08. If the sample size is multiplied by 9, what is the new margin of error?