  We study a noisy Kuramoto-Sivashinsky (KS) equation which describes unstable
surface growth and chemical turbulence. It has been conjectured that the
universal long-wavelength behavior of the equation, which is characterized by
scale-dependent parameters, is described by a Kardar-Parisi-Zhang (KPZ)
equation. We consider this conjecture by analyzing a renormalization-group
equation for a class of generalized KPZ equations. We then uniquely determine
the parameter values of the KPZ equation that most effectively describes the
universal long-wavelength behavior of the noisy KS equation.
