AP Statistics / Exploring One-Variable Data and Collecting Data
Lesson
Collecting data without stacking the deck
Sampling, bias, and the difference between watching and assigning treatments.
Learning goals
- Name a sampling method and the bias it can create.
- Explain why random assignment and random sampling answer different questions.
Explanation
A sample is useful when every unit in the population had a known chance to be chosen. Convenience samples and voluntary response surveys often over-represent people who are nearby or already motivated. That is bias: a systematic lean, not just unlucky noise.
Watching people is not the same as assigning them. An observational study can show association. An experiment randomly assigns treatments so that lurking differences are mixed across groups. Only then is a causal sentence usually fair.
Key terms
- Bias. A systematic tendency for a study to miss the truth in one direction.
- Random assignment. Placing experimental units into treatment groups by chance so groups start comparable.
Common mistakes
- Saying a large convenience sample is automatically trustworthy.
- Claiming cause from an observational study because the correlation looks strong.
Practice
Original Marlow Works items. Check the answer explanation after you try.
A reporter stands at one grocery-store exit and asks shoppers whether they support a new park tax. Why is this a poor way to estimate citywide support?
Original Marlow Works item — not a College Board question.
Take your time—this is practice, not a test.
Answer explanation
Researchers randomly assign 80 runners to two shoe types and measure 5K times. What kind of conclusion can they support if one shoe is faster?
Answer. A causal conclusion about the shoes for runners like those in the study, because treatments were assigned at random.
Random assignment balances lurking variables. Generalizing to all runners still depends on how the 80 people were chosen.
Related resources
External links with reuse status. Marlow Works is independent and does not copy restricted exam or textbook material.
Official / link only
AP Statistics course page
Official revised 2026–27 five-unit framework.
College Board · All rights reserved · accessed 2026-10-01
Open sourceCC BY — attribution required
Collaborative Statistics
Open statistics text for distributions, probability, and inference.
Open Textbook Library listing · CC BY · accessed 2026-10-01
Open source