Handout on Finding the Sample Size Needed for a Confidence Interval for a Single Population Proportion Table of z-values for commonly used confidence levels Suppose you want to construct a 95% confidence interval for the proportion of college students who floss daily.  You want a margin of error of no more than plus or minus 3%.  According to the ADA 12% of all Americans floss daily.  If you think that college students' flossing habits are similar to the general population of Americans, how many college students should you survey? Solution 15% of all homes in California will collapse in a major earthquake.  I you want to construct a 99% confidence interval for the proportion of all homes in El Dorado county that will collapse in a major earthquake such that the margin of error is no more than plus or minus 2%, how many homes must you include in your study? Last week, you surveyed 100 college students and found that 78% of them went to at least one party in the last 7 days.  If you want a margin of error of no more than plus or minus 4%, how many more students do you need to survey in order to construct a 95% confidence interval for the proportion of all college students who went to at least one party in the last 7 days? Suppose you want to construct a 90% confidence interval for the proportion of college students that want to work in the health care profession after graduation.  If you want a margin of error of no more than 5%, how many college students must you survey? Solution Suppose you want to construct an 80% confidence interval for the proportion of business that provide incentives to carpool to work.  If you want a margin of error of no more than 3.5%, how many businesses must you survey? If you want to find  a 95% confidence interval for the proportion of Californians who would consider taking a high speed train for traveling if there were a station in their neighborhood and you want a margin of error of no more than plus or minus 3%, how many Californians must you survey?