top of page

## Chapter 3 - Day 9

##### Common Student Errors
• Students will often flip the calculation for a residual by subtracting the actual y from the predicted y. Remind them that residual = When students are asked to describe the relationship between two quantitative variables, Actual – Predicted = A – P.

• When interpreting the slope of the equation for the least-squares regression line, students will often forget to include “predicted” when referring to the y variable.

• When interpreting the y-intercept of the equation for the least-squares regression line, students will often forget to include “predicted” when referring to the y variable value when x = 0.

• Students often think that outliers in a scatterplot must always have a big impact on the slope of the least-squares regression line. Remember that an outlier with an x-value close to the mean of x will pull the least-squares regression line towards itself, but without changing the slope much.

• When justifying why a linear model is appropriate for a set of two-variable data, students often want to give one single reason (the residual plot has no leftover pattern). Urge them to provide additional justification (r2 close to 1, small s).

• Students might have trouble memorizing the word-for-word interpretations of s and r2. We think this is OK as long as the meaning of what they write down is correct.

bottom of page