 Assuming weak separability of consumer preferences, the optimal allocation of expenditure among the brands of a given product category can be determined independently of the allocation of expenditure within other product categories.An extension of the almost ideal demand system is the Quadratic Almost Ideal Demand System (QUAIDS) which was developed by James Banks, Richard Blundell, and Arthur Lewbel.It considers the existence of non-linear engel curve which is not expressed in the standard almost ideal demand system.