I have a set of predictor variables (X1, X2...XN) and a single continuous response variable (Y) that I would like to build a prediction model from. The Y in this case, is the Volume of material but it is often expressed in units such as Volume per inch of rain, per foot of length. During the course of my sampling, we will be measuring both the volume, inches of rain, and the length.
My question is: Should these two 'denominator values' be used as predictors (Xs) and the Y is just the volume, or instead, do we first calculate the Y as Vol/rain*feet and use that as the response?
i.e.: is Vol = f(X1, X2, X-rain, X-length) or is it: Vol /(X-rain * X-length) = f(X1, X2)