Most binary vapor-liquid data are reported as P-x-y data sets. Barker's method, the conventional procedure for extracting the adjustable parameters of an activity coefficient model, utilizes just the P-x data component. An alternative but analogous procedure relies on experimental y-x information and is based on the dimensionless group Psi = alpha P-2*/P-1*. In this approach data points at the composition limits are more heavily weighed than those in the mid-range. It is found that Psi = Psi(x(1)) is nearly temperature invariant for non-ideal systems that contain an associating component. This suggests the utility of temperature-independent model parameters and facilitates their estimation.