In the long run, every input — plant size included — is variable. The LRATC curve shows the lowest possible average cost of producing each output level once the firm can choose any scale of operation. It is the "envelope" hugging the bottoms of all the possible short-run ATC curves.
[GRAPH: LRATC. X-axis "Quantity", Y-axis "Cost per unit". A wide U-shaped curve formed as the lower envelope of several overlapping short-run ATC curves. Left region labeled "Economies of scale (LRATC falling)"; flat middle labeled "Constant returns to scale"; right region labeled "Diseconomies of scale (LRATC rising)". The lowest output at which LRATC reaches its minimum is labeled "Minimum efficient scale (MES)".]
| Region | LRATC | Why |
|---|---|---|
| Economies of scale | Falling as Q rises | Specialization of labor and equipment, bulk purchasing, spreading design and setup costs |
| Constant returns to scale | Flat | Doubling all inputs doubles output — essentially replicating the operation |
| Diseconomies of scale | Rising as Q rises | Management layers, communication and coordination costs, bureaucracy |
Minimum efficient scale (MES): the smallest output at which LRATC reaches its minimum — the first quantity at the bottom of the curve, not the last. When MES is huge relative to market demand, only a few firms can operate efficiently (the road to concentrated markets and natural monopoly, later lessons). Tiny MES leaves room for many small firms.
Don't confuse two "things get worse" ideas: - Diminishing marginal returns — short run, requires a fixed input (Lesson 7). - Diseconomies of scale — long run, all inputs increase together. The CLEP exam plants this exact distractor.
Consider the adult-life decision this lesson is really about: quitting a salaried job to start a business.
Economic cost = explicit + implicit.
Accounting profit = Total revenue − explicit costs
Economic profit = Total revenue − explicit costs − implicit costs
Economic profit is never larger than accounting profit (implicit costs can't be negative).
Worked example: Dana quits a $65,000 job and invests $50,000 of savings (previously earning 4% interest) to open a consulting firm. Year one: revenue $190,000; explicit costs $95,000. - Accounting profit = 190,000 − 95,000 = $95,000 - Implicit costs = 65,000 forgone salary + 2,000 forgone interest = $67,000 - Economic profit = 95,000 − 67,000 = $28,000 — Dana beats her best alternative by $28,000.
Reading tip: in any profit problem, underline every "quit," "gave up," or "could have earned" phrase — that is your implicit-cost list.
Zero economic profit = normal profit. Revenue covers everything, including the owner's next-best alternatives — the owner does exactly as well as she could anywhere else. That is why zero economic profit is the long-run resting point of competitive markets (Lesson 9):
Q1 — D. Economies of scale is a long-run concept: LRATC falls as the firm's whole scale grows. A is the short-run AFC-spreading effect — the classic confusion, since both make average cost fall. B is a short-run productivity effect, not a scale property. C: total cost virtually has to rise with output; scale economies are about cost per unit. E: profit says nothing about the shape of the cost curve. Fix: "Economies of scale" lives on the LRATC curve — if the sentence is about fixed costs or the short run, it's the wrong concept.
Q2 — A. Implicit costs are the opportunity costs of owner-supplied resources: forgone salary, forgone interest, forgone rent. B describes explicit costs. C describes fixed costs — a different classification axis entirely. D: taxes and fees are explicit money payments. E reverses reality — implicit costs are precisely the ones that don't appear on financial statements. Fix: No cash changed hands but something was given up → implicit cost.
Q3 — D. Accounting: 190,000 − 95,000 = $95,000. Implicit: 65,000 salary + (0.04 × 50,000) = 67,000. Economic: 95,000 − 67,000 = $28,000. A ignores implicit costs for both measures. B swaps the two labels. C forgets the forgone interest ($2,000). E wrongly subtracts the forgone salary from accounting profit too. Fix: Compute accounting profit first (revenue − explicit only), then subtract all the "gave up" items once to get economic profit.
Q4 — B. Zero economic profit means revenue covered explicit costs and every opportunity cost, including her forgone salary — she matched her best alternative exactly. That is normal profit. A treats zero economic profit as failure; it's the healthy long-run norm. C reverses the definition — economic profit is the measure that includes opportunity costs. D: accounting profit will be positive (equal to her implicit costs), not zero. E: her implicit "pay" is fully covered — that's the point. Fix: Zero economic profit = "doing as well as your best alternative" — read it as a tie, not a loss.
Q5 — C. Rising LRATC in a firm that has scaled all inputs (500 locations) signals diseconomies of scale, whose textbook driver is management, communication, and coordination breakdown. A is a short-run concept requiring a fixed input — with 500 locations, everything has been varied. B: AFC falls with output; it cannot rise. D: industry-wide input price changes shift cost curves for everyone but are not diseconomies of scale. E: spreading overhead lowers average cost. Fix: All inputs grew and per-unit cost rose → diseconomies of scale; fixed input present → think diminishing returns instead.
Q6 — B. The defining difference: diminishing returns needs a fixed input (short run); diseconomies of scale happens when all inputs expand (long run). A states the exact opposite. C: the law applies to any variable input, most classically labor. D: both phenomena raise average cost in their own horizon. E: the law holds in any market structure. Fix: Fixed input present → "returns"; all inputs changing → "scale."
Q7 — E. MES is by definition the first (smallest) output at which LRATC hits its minimum. A describes the minimum of short-run ATC (where MC crosses it). B: AFC never stops declining. C: zero economic profit is a market outcome, not a cost-curve landmark. D: MES is the smallest efficient scale, not the largest feasible one. Fix: MES = the first quantity at the bottom of the LRATC curve.
Q8 — E. Minimum LRATC is $35, first reached at Q = 400 → MES = 400. A is where LRATC is still falling. B is the last quantity at the minimum, not the smallest. C is in the diseconomies region. D confuses MES (a single smallest quantity) with the entire flat constant-returns range. Fix: Find the lowest LRATC value, then pick the smallest Q that achieves it.
Q9 — C. If a firm must be enormous (relative to the market) to reach minimum cost, only a few firms can fit — the market concentrates. A: many small firms would all be stuck on the expensive falling portion of LRATC and be undercut. B: "short run only" isn't a market structure outcome. D: concentration tends to protect profit, not destroy it. E: the market is served — just by few firms. Fix: Compare MES to market demand: big MES → few firms; small MES → many firms.
Q10 — D. A flat LRATC over that range means scaling all inputs up changes output proportionally — constant returns to scale. A is the falling region below 1 million. B is the rising region beyond 3 million. C is a short-run concept — no fixed input is implied here. E describes rising MP, a short-run production idea, not a flat long-run cost region. Fix: Falling LRATC = economies; flat = constant returns; rising = diseconomies — match the slope, not the level.
Q11 — A. Economic profit = 40,000 − 55,000 = −$15,000: Marcus earns $15,000 less than his resources would in their best alternative use, so long-run exit pressure applies. B is the central trap — positive accounting profit can hide an economic loss. C: implicit costs are real opportunity costs even without cash outlays. D: expanding doesn't fix a situation where the owner's alternatives pay better. E: normal profit requires economic profit of exactly zero, not −$15,000. Fix: Long-run stay/exit decisions run on economic profit; accounting profit alone can't answer them.
Q12 — C. Accounting profit = 200,000 − 130,000 = $70,000, but the implicit cost of quitting is her $80,000 salary: economic profit = −$10,000, so on the numbers alone she should keep the job. A stops at accounting profit and ignores the forgone salary. B is even weaker — revenue exceeding explicit costs guarantees nothing. D misuses "normal profit": that would require economic profit of zero, and $70,000 < $80,000. E misapplies sunk cost — her salary is a forgone alternative going forward, the very definition of an opportunity cost, not a sunk cost. Fix: Before any "quit the job" decision, subtract the salary you'd give up; only a positive remainder argues for the switch.
Q1 — D. Economies of scale is a long-run concept: LRATC falls as the firm's whole scale grows. A is the short-run AFC-spreading effect — the classic confusion, since both make average cost fall. B is a short-run productivity effect, not a scale property. C: total cost virtually has to rise with output; scale economies are about cost per unit. E: profit says nothing about the shape of the cost curve. Fix: "Economies of scale" lives on the LRATC curve — if the sentence is about fixed costs or the short run, it's the wrong concept.
Q2 — A. Implicit costs are the opportunity costs of owner-supplied resources: forgone salary, forgone interest, forgone rent. B describes explicit costs. C describes fixed costs — a different classification axis entirely. D: taxes and fees are explicit money payments. E reverses reality — implicit costs are precisely the ones that don't appear on financial statements. Fix: No cash changed hands but something was given up → implicit cost.
Q3 — D. Accounting: 190,000 − 95,000 = $95,000. Implicit: 65,000 salary + (0.04 × 50,000) = 67,000. Economic: 95,000 − 67,000 = $28,000. A ignores implicit costs for both measures. B swaps the two labels. C forgets the forgone interest ($2,000). E wrongly subtracts the forgone salary from accounting profit too. Fix: Compute accounting profit first (revenue − explicit only), then subtract all the "gave up" items once to get economic profit.
Q4 — B. Zero economic profit means revenue covered explicit costs and every opportunity cost, including her forgone salary — she matched her best alternative exactly. That is normal profit. A treats zero economic profit as failure; it's the healthy long-run norm. C reverses the definition — economic profit is the measure that includes opportunity costs. D: accounting profit will be positive (equal to her implicit costs), not zero. E: her implicit "pay" is fully covered — that's the point. Fix: Zero economic profit = "doing as well as your best alternative" — read it as a tie, not a loss.
Q5 — C. Rising LRATC in a firm that has scaled all inputs (500 locations) signals diseconomies of scale, whose textbook driver is management, communication, and coordination breakdown. A is a short-run concept requiring a fixed input — with 500 locations, everything has been varied. B: AFC falls with output; it cannot rise. D: industry-wide input price changes shift cost curves for everyone but are not diseconomies of scale. E: spreading overhead lowers average cost. Fix: All inputs grew and per-unit cost rose → diseconomies of scale; fixed input present → think diminishing returns instead.
Q6 — B. The defining difference: diminishing returns needs a fixed input (short run); diseconomies of scale happens when all inputs expand (long run). A states the exact opposite. C: the law applies to any variable input, most classically labor. D: both phenomena raise average cost in their own horizon. E: the law holds in any market structure. Fix: Fixed input present → "returns"; all inputs changing → "scale."
Q7 — E. MES is by definition the first (smallest) output at which LRATC hits its minimum. A describes the minimum of short-run ATC (where MC crosses it). B: AFC never stops declining. C: zero economic profit is a market outcome, not a cost-curve landmark. D: MES is the smallest efficient scale, not the largest feasible one. Fix: MES = the first quantity at the bottom of the LRATC curve.
Q8 — E. Minimum LRATC is $35, first reached at Q = 400 → MES = 400. A is where LRATC is still falling. B is the last quantity at the minimum, not the smallest. C is in the diseconomies region. D confuses MES (a single smallest quantity) with the entire flat constant-returns range. Fix: Find the lowest LRATC value, then pick the smallest Q that achieves it.
Q9 — C. If a firm must be enormous (relative to the market) to reach minimum cost, only a few firms can fit — the market concentrates. A: many small firms would all be stuck on the expensive falling portion of LRATC and be undercut. B: "short run only" isn't a market structure outcome. D: concentration tends to protect profit, not destroy it. E: the market is served — just by few firms. Fix: Compare MES to market demand: big MES → few firms; small MES → many firms.
Q10 — D. A flat LRATC over that range means scaling all inputs up changes output proportionally — constant returns to scale. A is the falling region below 1 million. B is the rising region beyond 3 million. C is a short-run concept — no fixed input is implied here. E describes rising MP, a short-run production idea, not a flat long-run cost region. Fix: Falling LRATC = economies; flat = constant returns; rising = diseconomies — match the slope, not the level.
Q11 — A. Economic profit = 40,000 − 55,000 = −$15,000: Marcus earns $15,000 less than his resources would in their best alternative use, so long-run exit pressure applies. B is the central trap — positive accounting profit can hide an economic loss. C: implicit costs are real opportunity costs even without cash outlays. D: expanding doesn't fix a situation where the owner's alternatives pay better. E: normal profit requires economic profit of exactly zero, not −$15,000. Fix: Long-run stay/exit decisions run on economic profit; accounting profit alone can't answer them.
Q12 — C. Accounting profit = 200,000 − 130,000 = $70,000, but the implicit cost of quitting is her $80,000 salary: economic profit = −$10,000, so on the numbers alone she should keep the job. A stops at accounting profit and ignores the forgone salary. B is even weaker — revenue exceeding explicit costs guarantees nothing. D misuses "normal profit": that would require economic profit of zero, and $70,000 < $80,000. E misapplies sunk cost — her salary is a forgone alternative going forward, the very definition of an opportunity cost, not a sunk cost. Fix: Before any "quit the job" decision, subtract the salary you'd give up; only a positive remainder argues for the switch.