Lines of Fit
Concept

Residual

A residual is the vertical distance between a data point and the line of fit. When a line of fit has been drawn on a scatter plot, not all of the data points lie exactly on the line — some of them are above the line and some below. Therefore, each data point has one residual, which can be positive, negative, or zero.
Showing positive, negating, and zero residuals for a line of fit

A residual can also be defined as the observed y-value of a data point minus its predicted y-value, found using the line of fit.


Residual = lObserved y-value - lPredicted y-value

Generally, the smaller the absolute values of the residuals, the more reliable the line of fit is. A scatter plot of the residuals can be used to determine how well a model fits data set. The independent variable and the residuals are graphed as ordered pairs (x,residual).
The appropriate models can be determined by looking at the scatter plot of residuals.
  • If the points in the plot are randomly placed about the x-axis, then a linear model describes the data set well.
  • If some kind of pattern appears in the scatter plot, a non-linear model is more appropriate for the data.
Exercises