Occasionaly the relation between and is given by a table with data. If the data (almost) fits a straight line, you can formulate a linear model. To describe the linear model you would use a linear formula such as .
Suppose the straight line goes through the points and , you would find the linear formula as follows:
increases with = 20;
increases with = 90;
if were to increase with , would increase with ;
the gradient of the line is, therefore ;
the formula becomes ;
is on the line, so , which gives .
The straight line corresponds to the formula: .