where k is the number of regression coefficients. Based on Eqs. (2) and (4), it can be noted that k ¼ pþ1. In Eq. (6), the term np1 is used to maintain consistency with format found in the literature, thus it can also be noted that np1 ¼ nk.
The root mean square error (RMSE) of the model is another parameter to measure the quality of the fitting, which is a measure of the scatter in the data around the model. The RMSE for a simple linear model is computed as