Long-Run Costs & Economies of Scale
- Distinguish the short run from the long run by which inputs are variable
- Identify economies, constant returns and diseconomies of scale on the LRATC curve
- Explain why diseconomies of scale differ from diminishing returns
The long run is defined by what can change
The short run is any period in which at least one input is fixed — usually plant and equipment. The long run is the period in which all inputs are variable, so the firm can choose its scale, and firms can also enter or leave the industry. The distinction is not a fixed number of months; it varies by industry, from weeks for a food truck to a decade for a nuclear plant.
The LRATC curve as an envelope
Each possible plant size has its own short-run ATC curve. The long-run average total cost curve is the lower envelope of all of them — for each output, the cost of the best plant size for that output. So LRATC can never lie above any SRATC curve, and it touches each one at the output that plant is best suited to.
Where economies and diseconomies come from
Economies of scale come from specialization of labor and equipment, spreading indivisible costs like research and machinery, and bulk purchasing. Diseconomies of scale come from coordination and communication costs, layers of management, and weakening incentives as individual contribution becomes harder to observe. Both are long-run: nothing is fixed, and the firm is choosing a different scale rather than crowding an existing plant.
The distinction the exam tests
Diminishing returns is short-run: a variable input crowds a fixed one, so marginal product falls. Diseconomies of scale is long-run: the firm has scaled everything up and average cost still rises, because managing a larger organization is harder. A question describing "adding workers to an existing factory" wants diminishing returns; one describing "building a second, larger factory" wants scale.
A firm's LRATC is $40 at 100 units, $30 at 200 units, $30 at 300 units and $36 at 400 units. Identify the scale characteristics across the range and the minimum efficient scale.
- 1.From 100 to 200 units, LRATC falls from $40 to $30: economies of scale.
- 2.From 200 to 300 units, LRATC is flat at $30: constant returns to scale.
- 3.From 300 to 400 units, LRATC rises to $36: diseconomies of scale.
- 4.Minimum efficient scale is 200 units — the smallest output achieving the lowest LRATC.
LRATC being U-shaped is not the same statement as SRATC being U-shaped, and the two curves have different causes. SRATC turns up because of diminishing returns; LRATC turns up because of diseconomies of scale.
A firm doubles all its inputs and finds that average total cost falls. The firm is experiencing:
The long-run average total cost curve:
A conglomerate finds that as it grows, layers of management slow decisions and average cost rises. This is:
Read whether the scenario holds an input fixed. "More workers in the same plant" is short-run diminishing returns; "a larger plant" or "all inputs doubled" is long-run scale. The two have different answers and the wording is the only clue.
Answer the 3 checkpoints as you read.
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