An extension of the concept of response reactions (RERs) is presented that allows the generation of thermochemical data from group additivity (GA) and ab initio methods using precisely the same stoichiometric and mathematical procedures. This goal is achieved by (i) reformulating the ordinary least squares (OLS) analysis in terms of GA reactions, (ii) realizing that the modified OLS analysis is equally applied to both GA and ab initio methods, and (iii) proving that the errors (residuals) may be uniquely partitioned into a sum of contributions associated with GA RERs. These new findings provide a remarkable interpretation and direct comparison of the accuracies and capabilities of the GA and ab initio methods.