Econometric modelling has the advantage of measuring the relationships between economic variables empirically, which may include many interactions that have not been accounted for in a simple theoretical framework. In a second best world, a model that relies on first best theory may introduce large errors. Energy markets are often highly distorted and price responses vary between both countries and fuels. It has been shown that carbon taxes may have counterintuitive results due to complexities in fuel markets, even in some cases leading to an increase in CO2 per unit of energy. There are large differences in the responses made in the countries studied which appear difficult to reconcile with the assumption behind economic optimality of a tax.