Scatter plots are often used to see if an association, or correlation, exists between two variables.

If the data points are clustered about a line or curve, they have a **linear** or **nonlinear association**, respectively.

linear association | | linear association |

nonlinear association | | no association |

If a linear association exists, an equation can be developed to model the relationship.

The equation can be used to make predictions about the data.

The equation is derived from a straight line drawn to fit the data.

This line is referred to as a trend line, line of best fit, and/or regression line, and it can be used to make predictions outside of the data set.

The number of classes is shown on the *x*-axis, and the cost per class is shown on the *y*-axis.

Use the best fit line on the scatter plot to find the *x*-value when *y* equals 10.

The *x*-value of the point on the line where *y* equals 10 is about 38.

So, the best estimate of the number of classes that must be purchased for each class to cost $10 is 38 classes.