Since the demand function shows a relationship between the quantity demanded and the price, then a change in the price will be represented by a movement along the demand curve. Any other factor that influences the consumer's purchasing decisions, other than the own price, will shift the demand curve to the left or to the right.

Factors causing the demand curve to shift to the right (increase in demand)

• Increase in the price of a substitute good
• Increase in the consumer's income
• Increase in population
• A decrease in the price of a complement good

Factors causing the demand curve to shift to the left (decrease in demand)

• Decrease in the price of a substitute good
• Decrease in the consumer's income
• Decrease in population
• An increase in the price of a complement good