CB2400 Chap.10 Long-run Costs and Scale
Long-run Costs and Scale
Every input can adjust
In the long run the firm can vary all relevant inputs and choose the least-cost combination for each output. Long-run total cost records those minimum expenditures.
Long-run average cost divides that total by output, and long-run marginal cost measures the change in the minimum total when production expands.
Scale is classified by average cost
Falling long-run average cost indicates economies of scale, a flat region indicates constant average cost, and rising long-run average cost indicates diseconomies.
Total cost can rise while average cost falls if output grows proportionally faster than expenditure. Marginal cost below LAC pulls the average down, while marginal cost above it pushes the average up.
Technology provides the underlying reason
Increasing returns to scale means that a common percentage increase in all inputs produces a larger percentage increase in output.
With input prices fixed, this tends to lower average cost. Decreasing returns tends to raise it. Each point on long-run total cost can be understood as a separate isoquant-isocost minimisation at a different output level.
What this chapter covers
- 01
long-run flexibility
- 02
long-run total cost
- 03
long-run average cost
- 04
long-run marginal cost
- 05
economies of scale
- 06
returns to scale
Worked example · free
Classify scale from a cost schedule
- 3Divide each long-run total cost by its output.
- 2Compare the first two averages.
- 3Compare the second and third averages and name both regions.
Key terms
- Long-run Total Cost
- The minimum total expenditure needed for each output when all inputs can adjust.
- Long-run Average Cost
- Minimum long-run total cost divided by the associated output.
- Long-run Marginal Cost
- The change in minimum long-run total cost from additional output.
- Economies Of Scale
- A range where long-run average cost falls as output increases.
- Diseconomies Of Scale
- A range where long-run average cost rises as output increases.
Long-run Costs and Scale FAQ
Can total cost rise during economies of scale?
Yes. Economies of scale means long-run average cost falls, not that the total bill falls. If output grows faster proportionally than long-run total cost, cost per unit declines even though more inputs are purchased and aggregate expenditure increases.
How do returns to scale differ from economies of scale?
Returns to scale is a production relationship: it compares the percentage output response when all inputs change proportionally. Economies of scale is a cost relationship: it concerns long-run average cost as output changes. With input prices fixed, increasing returns tends to support economies of scale, but the concepts are not identical definitions.
What does LMC below LAC imply?
The next output increment costs less than the current long-run average, so it pulls that average downward. If LMC rises above LAC, the average is pushed upward. Their smooth-curve crossing therefore occurs at the minimum of LAC, following the same marginal-average logic used for short-run costs.
Exam move
For every scale problem, write LTC, LAC and LMC on separate lines and include units. Compare percentage changes when totals are supplied, then confirm with cost per unit. Link the result back to input flexibility: each long-run cost point is the output of a least-cost producer-choice problem, not a calendar forecast. Construct a cost schedule with several output levels and compute LAC in every row.
Classify adjacent intervals rather than jumping from the first observation to the last, because a falling region followed by a rising region can disappear in an overall average. Calculate arc LMC from the change in LTC divided by the change in output and use the marginal-average rule to audit the LAC direction. When LMC is below LAC, the average should fall; when it is above, the average should rise.
Keep production and cost language distinct. Increasing returns to scale compares a proportional increase in all inputs with the output response. Economies of scale describes falling long-run average cost. With input prices fixed the concepts are linked, but they are not interchangeable definitions.
End by identifying what all-input flexibility changes relative to the short run and why no universal calendar duration defines the horizon. Mark the lowest observed LAC without assuming it is the exact smooth-curve minimum, since a discrete schedule may omit cheaper outputs between listed rows.
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