MetrixND provides several options for determining whether a variable is useful. Typically, a model builder reviews the variable coefficients, standard error, t-statistic and p-values. These statistics are located on the “Coef” tab as shown below.

bxtab1

 

 

 

 

An evaluation of the variable coefficients shows that each variable is significant and behaves as expected. For instance, the CDD70 variable has a t-statistic of 11.563 (statistically significant) and indicates that when cooling degree days (CDD70) increase, sales increase by 1.896 units.

While these numerical results are useful, a graphical depiction of the variable aids the understanding of the variable’s contribution to the model. In the regression model object, the BX tab provides the picture of each explanatory variable’s contribution to the predicted value.

In the figure below, the contribution of CDD70 (red line) is compared to the model’s predicted value (blue line). Formally, the CDD70 line is calculated from the CDD70 variable value and the CDD70 coefficient (BCDD70 x XCDD70).

bxtab2

 

 

 

 

 

 

 

 

 

 

The power of the graphic is visual clue about what each variable is doing to generate the predicted value. In this example, the CDD70 variable is responsible for the summer cooling shape.

A closer inspection of the graphic (zooming into 2014 in the figure below), shows how CDD70 only contributes to the predicated values in May through October. So while the predicted value decreases from April to May, the graphic tells us the decrease is not caused by the CDD70 variable because CDD70 does not contribute to the April predicted value.

bxtab3

 

 

 

 

 

 

 

 

 

 

Coupled with the traditional variable statistics, the BX view enhances the model builder’s understanding of variable’s contribution and power. Begin using the BX tab and add it to your box of model building tools.