Advertising expenditure must remain the same
If advertising expenditure of a firm increases, consumers may be tempted to buy more of its product. Hence the advertising expenditure on the good under consideration is taken to be constant. Desire of a person to buy a commodity isn't his demand. He should possess adequate resources and should be willing to spend his resources to buy the commodity. Moreover the quantity demanded has always a reference to 'a price' and 'a unity of time'.