Cross-price elasticity of demand is a measure of consumers responsiveness in demand for a product when the price of a related product changes. This formula determines whether goods are substitutes, complements or unrelated goods. In order to find this figure, you must INCLUDE negative values into the formula. 

• Complementary goods describe products that are bought together, for instance, socks are commonly bought with sneakers. These goods have an inverse relationship, meaning, if the price rises for one good, the demand will fall for both goods. 

• Substitute goods are alternatives for each other. These goods have a positive relationship. For instance, if the price rises for orange juice, consumers can choose to purchase apple juice instead, therefore, raising the demand for apple juice.