It’s hard to overstate the importance of understanding the difference between shifts in curves and movements along curves. Remember, when we talk about changes in demand or supply, we do not mean the same thing as changes in quantity demanded or quantity supplied.

A change in demand refers to a shift in the entire demand curve, which is caused by a variety of factors (preferences, income, prices of substitutes and complements, expectations, population, etc.). In this case, the entire demand curve moves left or right.

Figure 1. Change in Demand. A change in demand means that the entire demand curve shifts either left or right. The initial demand curve D₀ shifts to become either D₁ or D₂. This could be caused by a shift in tastes, changes in population, changes in income, prices of substitute or complement goods, or changes future expectations.

A change in quantity demanded refers to a movement along the demand curve, which is caused only by a chance in price. In this case, the demand curve doesn’t move; rather, we move along the existing demand curve.

Figure 2. Change in Quantity Demanded. A change in the quantity demanded refers to movement along the existing demand curve, D₀. This is a change in price, which is caused by a shift in the supply curve.