Common Mistakes
Statistics · AP · Exploring One-Variable Data
Focus
Summary Statistics for a Quantitative Variable
This topic is assessed on the AP Exam.
Frequent mistakes
### 1. Mean on skewed data
What it looks/sounds like: Mean is reported as ‘typical’ when distribution is skewed.
Why it happens: Mean and median are interchangeable in the student’s mind.
How to fix: Use median and IQR for skewed or outlier-heavy data.
### 2. Outlier overcall
What it looks/sounds like: Every far point is called an outlier without a rule.
Why it happens: Visual distance alone is used.
How to fix: Use 1.5×IQR or explain why a point is unusual in context.
### 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. Describe shape, center, spread, outliers in context. 2. Choose median/IQR vs mean/SD deliberately. 3. Use units on all summaries.
Quick self-check
- [ ] Can describe a distribution fairly
- [ ] Can pick median vs mean appropriately
- [ ] Can compare two distributions