Although economists emphasize the importance of price in purchasing decisions, they also recognize that a multitude of factors other than price affect the amount of a good or service people will purchase. Economists concentrate only on the most important factors. What factors can be considered sufficiently important to be included in most studies of market demand, and why do we have to keep all other factors constant while examining the effect of a change of one factor on the amount of a good or service people will purchase give examples.