Market demand adds individual quantities

Add horizontally at each common price.

If at $5 Ana buys 2 units and Bo buys 3, market quantity demanded is 5. Do not add their willingness-to-pay prices for a private good. Horizontal summation preserves the meaning of the axes: at one market price, how many total units will all buyers purchase?

Suppose a third buyer, Chen, buys 4 units at $5. Market quantity becomes 2+3+4=9. At $8, the three buyers might choose 1, 2, and 2 units, so market quantity is 5. Each price gets its own horizontal addition. Adding Ana’s quantity at $5 to Bo’s quantity at $8 would combine plans that cannot occur at one common market price.

For a private good, each unit is consumed by one buyer, so quantities add. Public goods use a different construction because many people can consume the same unit. Their marginal willingness to pay may be added vertically. An introductory question normally signals public-good valuation explicitly. In an ordinary product market, horizontal summation is the default.

Market demand can shift even when every individual’s demand curve stays fixed. Population growth or entry of new buyers adds quantities at each price. It can also shift when the composition of buyers changes. A town with more commuters may demand more gasoline even if each existing commuter’s preferences and income are unchanged.

The distinction between demand and quantity demanded matters for causal language. A rise in demand can raise both equilibrium price and quantity. A rise in quantity demanded is the response to a lower price, often caused by a supply increase. If a question says sales and price both rose, a demand increase is consistent. Movement along a fixed demand curve cannot make both rise.

Observed price and quantity together can help identify which curve probably shifted, under the basic single-shift model. Price and quantity moving in the same direction is consistent with a demand shift. Price and quantity moving in opposite directions is consistent with a supply shift. This inference is conditional: if both curves moved, more information is needed.

Build a market schedule

At a price of $4, three consumers demand 6, 3, and 1 units. Market quantity demanded is 10. At $6, they demand 4, 2, and 0 units. Market quantity is 6. The market demand curve connects (Q=10,P=4) and (Q=6,P=6) with other horizontally summed points. The zero is included: one consumer leaving at that price changes the sum but does not eliminate the market.

Do not confuse horizontal summation with averaging. The market does not demand the average person’s quantity. It demands the total quantity across buyers. Nor should prices be added for the same private unit, because one unit is not simultaneously purchased by all consumers. Keep the physical nature of the good in view when deciding what to sum.

When demand shifts, market equilibrium determines the final quantity purchased. A rightward demand shift does not guarantee that quantity bought rises by exactly the horizontal size of the shift because price may rise along supply. The shift describes buyers’ plans at each fixed price. The new intersection incorporates sellers’ response.

Use a blank before the noun

Ask whether the sentence should read “a change in ___ causes a change in quantity demanded” or “causes a change in demand.” The good’s own price fills the first blank. Income, tastes, expectations, buyers, and related-good prices fill the second.