A linear regression is a line used to estimate the relationship between a dependent and an independent variable for a data set. In a diagram with a data set showing the height, $h,$ of tomato plants after they have grown for $t$ weeks, the line estimates how fast the plants grow.

This line, called the line of fit, can be applied to a data set by hand with the help of a ruler. This typically results in only a rough estimate of the relationship. To get a more accurate estimate of the equation of the line of fit, other mathematical methods can be employed, such as the method of least squares.