*5.2. Answer*

Although Kumbhakar et al. (2012, p. 110) describe first the problem that the marginal cost (*MC*) is usually not observable, pointing out additional weaknesses of the traditional approaches in later pages, one of their innovations is indeed related to output price. As it can easily be seen in Figure 2, the estimation of our Equation (7) does not require the availability of separate information on output price, since information on total revenue is sufficient (Kumbhakar et al. 2012, p. 116).

#### **6. Description of Some Auxiliary Equations**
