Imagine you want to know what share of all voters support a policy, but you can’t ask every voter. You take a random sample and find that 62% of them support it. That single number is your best guess, but you know it’s probably not exactly the truth for the whole population. How can you put a range around that 62% that you are fairly sure contains the true percentage? That’s exactly what a confidence interval does—and in this chapter, we’ll build one for a single proportion.