Principles of Microeconomics / Unit 5 MIC 0419 worked example, 60 to 90 seconds
Summing Individual Demands
The idea
Given two or more individual demand schedules, build the market schedule row by row. Pick a price, read each consumer's quantity at that price, and add them; that sum is the market quantity at that price. Repeat for every price on the schedules. A consumer whose quantity is zero at a high price contributes nothing at that row and something below it, which is why the market curve is flatter and further right than any individual's.
In practice
Worked example: for apples at three dollars, Ana wants two pounds, Ben zero and Cal four, so the market wants six. At two dollars, Ana wants five, Ben three and Cal eight: sixteen. At one dollar, Ana wants nine, Ben seven and Cal twelve: twenty-eight. The market schedule is six, sixteen and twenty-eight pounds at three, two and one dollars.
Where people slip
Two errors. Averaging the quantities instead of adding them, which understates the market by the number of buyers. And adding the prices each buyer would pay for a given pound, which builds a willingness-to-pay stack rather than a demand curve; that vertical sum has a use in unit twelve, for public goods, and none here.
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