Elasticity
The percentage change in one variable in response to a percentage change in another — most commonly demand or supply with respect to price.
Income elasticity > 0 marks normal goods; > 1 marks luxuries; < 0 marks inferior goods. Cross-price elasticity > 0 indicates substitutes; < 0 indicates complements. The CFA exam often tests the relationship between elasticity and total revenue at different points along a linear demand curve.