Home / Courses / Introductory Statistics

Introductory Statistics

Free Introductory Statistics study guide: 9 units with summaries, key ideas, flashcards and 243 practice questions with step-by-step solutions.

Lines up with: AP Statistics (revised 2026-27 framework).

Units

1. UNIT 01

   Describing one-variable data

   Before any formulas, statistics starts with describing data: what kind of variable it is, what its distribution looks like, and where a value sits relative to the rest.
2. UNIT 02

   Collecting data: sampling & experiments

   How data is collected decides what you're allowed to conclude. Random sampling lets you generalize; random assignment lets you claim cause and effect.
3. UNIT 03

   Probability

   Probability measures how likely an event is in the long run. This unit gives you the rules for combining events.
4. UNIT 04

   Random variables & distributions

   A random variable assigns a number to each outcome of a chance process. Its distribution tells you the possible values and how likely each one is.
5. UNIT 05

   Sampling distributions

   A statistic changes from sample to sample. Its sampling distribution shows that variation, and it's the foundation for every confidence interval and test.
6. UNIT 06

   Inference for proportions

   Confidence intervals estimate a population proportion; significance tests check whether data give convincing evidence against a claimed value.
7. UNIT 07

   Chi-square tests

   Chi-square tests compare observed counts in categories to the counts you'd expect if the null hypothesis were true.
8. UNIT 08

   Inference for means

   When the data are measurements, you estimate or test a population mean with t-procedures, because the population SD is usually unknown.
9. UNIT 09

   Regression

   Regression models the relationship between two quantitative variables with a line, and measures how well the line fits.