In most cases the dependent variable used was total energy intensity (as opposed to the fossil fuel intensity in the initial model), including nuclear and hydro expressed as its fossil fuel replacement value. Although (by definition) increases in non-fossil fuel must be accompanied by decreases in fossil fuel for a given energy demand, this relationship was not always adequately represented by including the change in non-fossil fuel consumption in the endogonous trend. Using total energy consumption allows fossil fuel consumption to be calculated by subtracting exogenous forecasts of non-fossil fuel production.