A firm can choose among several possible plant sizes, each with its own U-shaped short-run average total cost (SRATC) curve. The long-run average cost (LRAC) at any output level represents:
AThe arithmetic average of all SRATC curves at that output level
BThe minimum cost achievable at that output level by selecting the optimal plant size
CThe single lowest SRATC curve, since the firm would always build the cheapest plant
DThe SRATC curve for the plant size designed to produce exactly that output most efficiently
The LRAC is the envelope of all SRATC curves — the minimum cost achievable at each output level across all possible plant sizes. At any given output, the firm selects the plant size that minimizes average cost, and the LRAC records that minimum. Option C is the classic misconception: the LRAC lies below every individual SRATC at most outputs, not equal to any single one of them. Each SRATC is cheapest only near its designed capacity; for other output levels, a different plant size would be cheaper. The envelope captures this by selecting the optimal plant separately at each output level.
Question 2 Multiple Choice
An industry's minimum efficient scale (MES) equals approximately 70% of total market demand. What market structure is this industry most likely to exhibit?
APerfect competition with many small firms each well below MES
BNatural monopoly or tight duopoly, since only one or two firms can operate at minimum cost
CMonopolistic competition, since high MES encourages product differentiation
DThe market structure depends entirely on demand elasticity, not on MES relative to demand
When MES is large relative to market demand, only one or two firms can simultaneously operate at minimum efficient scale. A second entrant would face much higher average costs than the incumbent, creating an insurmountable cost disadvantage. This is the defining condition for natural monopoly: economies of scale are so large that the market can only support one firm efficiently. Water utilities, power transmission networks, and railroad infrastructure are classic examples. When MES is small relative to demand, many firms can each reach minimum cost, enabling competitive market structure.
Question 3 True / False
The long-run average cost curve is the envelope of all short-run average total cost curves, meaning it can lie below every individual SRATC curve at most output levels.
TTrue
FFalse
Answer: True
This is precisely the definition of an envelope curve. The LRAC records the minimum of all SRATC values achievable by choosing the optimal plant size at each output level. Since each SRATC curve is only at its own minimum near its designed capacity, the LRAC lies below (or touches) each SRATC for most output levels — it is never above any SRATC and touches a particular SRATC only at one tangency point. Students often assume the LRAC simply is the lowest SRATC, but it is a new curve constructed by optimizing plant choice separately at every output.
Question 4 True / False
Diminishing marginal returns and diseconomies of scale are essentially the same economic concept — both describe rising costs as output increases.
TTrue
FFalse
Answer: False
These are distinct concepts applying to different time horizons. Diminishing marginal returns is short-run: with at least one fixed input (typically capital), adding more of the variable input (labor) eventually yields smaller output increments, raising marginal and average variable costs. Diseconomies of scale is long-run: when all inputs are variable and the firm expands every input proportionally, average costs eventually rise due to coordination failures and bureaucratic inefficiency at very large scale. Confusing the two leads to incorrect analysis — a firm can exhibit diminishing returns in the short run while still enjoying economies of scale at larger plant sizes in the long run.
Question 5 Short Answer
Explain the difference between the short-run concept of diminishing marginal returns and the long-run concept of diseconomies of scale, including what defines each time horizon.
Think about your answer, then reveal below.
Model answer: The short run is defined as the period when at least one input is fixed (typically capital). Diminishing marginal returns occurs because adding more of a variable input (labor) to a fixed capital stock eventually yields smaller output increments, driving up marginal cost. The long run is defined as the period when all inputs are variable — the firm can choose its plant size freely. Diseconomies of scale occur when expanding all inputs proportionally leads to rising average costs, typically due to coordination costs and management inefficiencies at very large scale. The key distinction: diminishing returns results from a fixed-input constraint; diseconomies of scale result from organizational problems that arise even when all inputs can be freely adjusted.
This distinction matters practically. A firm experiencing diminishing returns in the short run might solve the problem by expanding its plant in the long run — getting onto a lower SRATC curve. But if the long-run LRAC is also rising at that scale, expanding the plant won't help. Understanding which time horizon applies is essential for diagnosing whether rising costs are a short-run capacity constraint (solvable by investing in more capital) or a long-run structural problem (requiring a fundamentally different scale or organizational approach).