Which statistical test should I use?
Two questions, sometimes three. Answer them about your data and your design and the right procedure falls out, with the calculator to run it.
What kind of response variable do you have?
This is the variable you measured on each subject, the one your question is about.
It really is only two questions
Choosing a test comes down to what kind of data you have and how it was collected. Categorical responses, the ones you summarize with a proportion, lead to z tests and chi-square. Quantitative responses, the ones you summarize with a mean, lead to t tests and regression. After that you only need to know how many groups you have, and if there are two, whether they are paired.
The full reasoning, the quick-reference table, and the standard error for each procedure live in the decision guide. This tool is that guide with the questions asked one at a time.
Frequently asked questions
How do I know if my data is paired or independent?
Ask whether each observation in one group has a natural partner in the other. The same subjects measured twice, such as before and after, is paired. Subjects deliberately matched by a trait like age, with one assigned to each treatment, is paired. Two separate groups with no matching are independent, even when they happen to be the same size. This is the most common wrong turn, and it changes both your standard error and your degrees of freedom.
Why do proportions use z and means use t?
A proportion has no separately estimated standard deviation, because its spread is a fixed function of the proportion itself. A mean's standard error uses the sample standard deviation as an estimate, and that extra uncertainty is exactly what the t distribution accounts for.
Do I run a significance test on a regression slope?
Not on the Fall 2026 AP exam. Regression is descriptive there: you fit the line and interpret the slope, the correlation, and the residuals, but you do not test the slope. Inference for a regression slope was removed from the AP course effective Fall 2026, though it remains standard in college statistics courses.
What if I have more than two groups?
For categorical data arranged as a table of counts, that is a chi-square test. AP Statistics does not cover ANOVA, which is the usual procedure for comparing more than two means in a college course.
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