The mathematical purpose of this lesson is to informally assess the fit of a function by plotting and analyzing residuals. The term residual is introduced as the difference between the -value for a point in a scatter plot and the value predicted by the linear model for the associated -value.

When students take turns with a partner matching graphs of residuals to scatter plots that display linear models, students trade roles explaining their thinking and listening, which provides opportunities for them to explain their reasoning and critique the reasoning of others (MP3).