Confidence Interval Estimation – Discussion

When the results of a survey or poll are published, the sample size and the margin of error are both given. For example: 1000 voters were surveyed and 39±2% of the voters agree with the president. In this example N=1000 and the margin of error (MoE) is 2%.

This website lists several public opinion polls. Search the site and find a poll where the sample size and margin of error are given. Try to find a poll dealing with a topic in your profession or one in which you are really interested.

http://www.pollingreport.com

Determine the following information for the selected poll results and include in your initial post:

1. URL for the website. State the poll question, the sample size N, and the margin of error (also known as sampling error).
2. Interpret the results of your poll using your own words and full sentences. Note: Depending on the question asked, your poll may have more than one poll result – you only need to discuss one result.
3. State the confidence interval using the MoE. What does this confidence interval estimate?
4. Use this worksheet (click to download CIE Proportion.xlsx) to calculate the confidence interval based on the sample size and the number of successes (the proportion you are interested in). Use a confidence level of 95%.
5. What is the calculated confidence interval? How does this compare to the interval in part 3 using the poll’s MoE?
6. Discuss potential biases that could skew sampling results.

See Example post.

First response: Choose a classmate’s post. Calculate the sample size needed for the given MoE and see if the poll had enough people surveyed. Use this worksheet (click to download Sample Size Proportion.xlsx) to find the sample size needed.

See Example post.

Second response: Choose another classmate’s post. Change the confidence level to 90% or 99%. What happened to the confidence interval (compared to your classmate’s)? Explain the change.

See Example post.