Since the processes involve important strain rates and moderate extensions, knowledge of the transient linear extensional response is enough for the description of the melt rheology. That is, knowledge of the linear properties is a good starting point. If extensional data are available, they can be used (either in the automatic fitting calculation, or for checking the results of the automatic fitting). The extensional viscosity can be measured for moderate strain rates (usually up to 10 s^-1 using an elongational rheometer equipped with the EVF device), although the material is often processed at a much higher strain rate.

Although shear is usually not an important component of the total deformation, shear viscosity is needed in the fitting procedure. This can be obtained from the Cox-Merz rule [4] (see Empirical Rules and Principles), and can be assigned a zero weight (see Weighting Measured Data).