Common Mistakes
Statistics · AP · Inference for Quantitative Data — Slopes
Focus
Confidence Intervals for the Slope of a Regression Model
This topic is assessed on the AP Exam.
Frequent mistakes
### 1. Inference without conditions
What it looks/sounds like: Calculator p-value or CI is reported with no plan step.
Why it happens: Conditions feel optional on technology items.
How to fix: Always state Random, 10%, and Large Counts/Normality as applicable.
### 2. Testing correlation not slope
What it looks/sounds like: Claims about correlation when inference is on β₁.
Why it happens: Output labels are misread.
How to fix: Focus inference language on the population slope.
### 3. Conditions skipped on regression
What it looks/sounds like: Linear regression inference without residual check.
Why it happens: Computer output is enough without conditions.
How to fix: Comment on linearity, independence, equal variance, Normal residuals.
### 4. 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.
### 5. 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 SE from computer output. 2. State regression inference conditions. 3. Write slope CI/conclusion in context.
Quick self-check
- [ ] Can interpret regression output
- [ ] Can state regression conditions
- [ ] Can write a slope conclusion