temperature is increased from x to x + 1 units, the expected yield changes by (This can be seen by obtaining the derivative with respect to x of the regression formula.) For infinitesimal changes in x, the effect on y is given by the total derivative with respect to x: The fact that the change in yield depends on x is what makes the relationship between x and y nonlinear even though the model is linear in the parameters to be estimated.

In general,