Inference for Categorical Data: Chi-Square — AP Statistics
1. Unit at a Glance
This unit builds on your existing inference knowledge to handle categorical data with more than two categories, or multiple groups. We start with core properties of the chi-square distribution, then work through three distinct chi-square procedures, ending with skill practice to select the correct test for any AP exam question.
All chi-square tests in this unit use the same general test statistic formula, but differ in their hypotheses, study design, and degrees of freedom. This distinction is the most heavily tested (and most commonly misunderstood) concept in this unit.
Common Pitfalls
Why: Both use two-way tables and identical calculations, but they answer different research questions
Why: The chi-square distribution is only a valid approximation when expected counts are large enough
Why: Students often mix up df formulas across different chi-square tests