Principles of Microeconomics / Unit 6 MIC 0542 review, 2 to 3 minutes
Why Industries Differ in Firm Size
The idea
Why do some industries hold thousands of small firms and others three giants, and why do some hold both? The long-run average cost curve answers all three, and the answer sets up the four market structures of the next units. Draw the curve for one industry. If it reaches its minimum at a small output and turns up soon after, minimum efficient scale is small, big firms pay for their size, and the market is filled by many small firms: restaurants and barbers. If the curve keeps falling out to an output that is a large share of the whole market, a single big firm undercuts every smaller rival, and the market holds a few firms or one: refrigerator makers, or the local water utility. If the curve is flat over a wide range, firms of every size within that range have the same unit cost, and the industry mixes them: one plant or four, both efficient. Market demand sets the other half of the answer. The same cost curve that supports a hundred firms in a large market supports three in a small one, so the count is the market's quantity at the minimum cost divided by the minimum efficient scale. And technology moves the curve over time: an invention that lowers the scale invites entry, one that raises it drives consolidation. The four structures of unit six's spectrum are, at bottom, four answers to how big a firm has to be to be cheap, set against how big the market is.
In practice
Worked example: three industries, one method. A city's restaurant trade turns over ten million dollars a year, and a restaurant's average cost bottoms at three hundred thousand of sales and rises above five hundred thousand: room for twenty to thirty restaurants, many small firms. A regional cement market takes a million tons a year, and a plant's average cost keeps falling to four hundred thousand tons: room for two or three plants, an oligopoly. A town's water demand is two million gallons a day, and a treatment plant's average cost is still falling at five million: one plant serves the whole town cheapest, a natural monopoly. Same curve-reading, three market structures.
Where people slip
Three cautions. The curve alone does not fix the count; it must be read against market size, which is why the same cement technology gives an oligopoly in one region and a single plant in a smaller one. Firms in the flat range are not inefficient for being different sizes; that is what constant returns means. And the structure is not permanent: a technology that halves minimum efficient scale can turn a monopoly into a competitive industry, which is a theme unit ten returns to when it asks whether a natural monopoly still is one.
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