Exercise 5: Confidence Intervals for Proportions

The Kaiser Family Foundation (KFF) COVID-19 Monitor conducts a respected survey on American's attitudes on the COVID-19 vaccine. KFF conducted a telephone survey on 1,862 adults 18 and older from March 15-22. Of the 1,862 respondents, 1,596 provided their party identification:

Here are the results for "eager to get the vaccine or had already received the vaccine."*

Party	%	n
Democrat	79%	577
Republican	46%	157
Independent	57%	298

A. What are the point estimates for Democrats, Republicans, and Independents?

Party	Point Estimate
Democrat
Republican
Independent

B. What is the appropriate z-value to use when constructing a confidence interval at the 95% and 99% confidence levels?

Confidence Level	z-value (Table)	z-Value (Excel)	Excel Formula
95% (0.95)			=ABS(NORM.S.INV(0.025))
99% (0.99)			=ABS(NORM.S.INV(0.005))

C. Calculate the three confidence intervals using a 95% confidence level. You may do this by hand or with Excel. You must show your work. Your answer must include the Point Estimate, Margin of Error (MoE), Lower Confidence Limit (LCL), and the Upper Confidence Limit (UCL). Excel does not have a Confidence Interval function for proportions. (Show your work. Use Excel.)

D. What do the confidence intervals you constructed tell you?

E. What would happen to the confidence intervals if you changed the level of confidence?

F. What would happen to the confidence interval if you increase the sample size?