A differential demand system for quasi separability is developed, following the approach used by Theil et al. in developing the Working-PI time series model. The Slutsky coefficients vary with commodity budget shares and intergroup substitution depends on group compensated price effects.

The differential demand system or Rotterdam model is extended to include lagged consumption through translation parameters, providing an alternative to simply adding constant terms to the model to allow for trends in consumption and changes in tastes. Application of the model to four broadly...