Common Mistakes
Statistics · AP · Collecting Data
Focus
Potential Problems with Sampling
This topic is assessed on the AP Exam.
Frequent mistakes
### 1. Causation from observation
What it looks/sounds like: Causal language is used for an observational study.
Why it happens: Association is confused with cause.
How to fix: Reserve causation for well-designed experiments.
### 2. Sampling bias
What it looks/sounds like: Convenience or voluntary samples are called ‘random’.
Why it happens: Sampling method vocabulary is loose.
How to fix: Name the method and how bias could enter.
### 3. Correct work, wrong question
What it looks/sounds like: The algebra is fine but answers a different quantity than the stem asks for.
Why it happens: Attention locks onto the first operation instead of the final ask.
How to fix: Underline the final question; after solving, verify the boxed answer matches it.
### 4. Notation and justification gaps
What it looks/sounds like: Missing units, limit language, or ‘because’ statements AP graders expect.
Why it happens: Notation is treated as decoration rather than part of the score.
How to fix: Add one justification phrase per major step (definition, theorem, or rule).
How to avoid them
1. Identify observational vs experimental design. 2. Explain what was randomized. 3. Name confounding when groups differ.
Quick self-check
- [ ] Can name the design and population
- [ ] Can explain randomization’s role
- [ ] Can avoid causal claims when inappropriate