A market supply curve represents the combined supply of all producers. An increase in the number of suppliers shifts the curve to the right (see Figure5.2) or to the left if suppliers leave the market. With a market supply curve we should assume those firms are producing an identical good. Whilst this might be a reasonable approach for relatively homogeneous goods such as potatoes or basic raw materials, we cannot meaningfully derive a market supply curve where firms are producing differentiated goods. For example, although we might measure total market car production, the derivation of a market supply curve presents difficulties.