  Production functions are potentially misspecified when revenue is used as a
proxy for output. I formalize and strengthen this common knowledge by showing
that neither the production function nor Hicks-neutral productivity can be
identified with such a revenue proxy. This result obtains when relaxing the
standard assumptions used in the literature to allow for imperfect competition.
It holds for a large class of production functions, including all commonly used
parametric forms. Among the prevalent approaches to address this issue, only
those that impose assumptions on the underlying demand system can possibly
identify the production function.
