Chapter 7
Confidence Intervals
IN THIS CHAPTER
Confidence interval components
Interpreting confidence intervals
Details of confidence intervals for one or two means/proportions
In this chapter, you find out how to build, calculate, and interpret confidence intervals, and you work through the formulas involving one or two population means or proportions. You also get the lowdown on some of the finer points of confidence intervals: what makes them narrow or wide, what makes you more or less confident in their results, and what they do and don’t measure.
Making Your Best Guesstimate
A confidence interval (abbreviated CI) is used for the purpose of estimating a population parameter (a single number that describes a population) by using statistics (numbers that describe a sample of data). For example, you might estimate the average household income (parameter) based on the average household income from a random sample of 1,000 homes (statistic). However, because sample results will vary (see Chapter 6), you need to add a measure of that variability to your estimate. This measure of variability is called the margin of error, the heart of a confidence interval. Your sample statistic, ...