Common Mistakes
Statistics · AP · Exploring Two-Variable Data
Focus
Representing Two Categorical Variables
This topic is assessed on the AP Exam.
Frequent mistakes
### 1. r confused with slope
What it looks/sounds like: Correlation is interpreted as the regression slope.
Why it happens: r and b are not distinguished.
How to fix: Slope b is change in y per unit x; r is correlation strength/direction.
### 2. Extrapolation
What it looks/sounds like: Predictions far outside x-range are treated as reliable.
Why it happens: The line is extended without caution.
How to fix: Note when prediction is extrapolation and less trustworthy.
### 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. Interpret slope and r² in full sentences. 2. Read residual plots for linearity. 3. State whether a prediction is interpolation.
Quick self-check
- [ ] Can interpret slope in context
- [ ] Can explain r² plainly
- [ ] Can spot nonlinearity in residuals