Chapter 17 — Key Takeaways
The cost categories
| Cost | Definition | Changes with output? | Examples |
|---|---|---|---|
| Fixed cost (FC) | Doesn't change with output | No | Rent, insurance, salaried staff, equipment payments |
| Variable cost (VC) | Changes with output | Yes | Hourly labor, raw materials, packaging, electricity |
| Total cost (TC) | FC + VC | Yes (rises with output) | |
| Average total cost (ATC) | TC/Q | U-shaped | |
| Average fixed cost (AFC) | FC/Q | Always declining (spreading overhead) | |
| Average variable cost (AVC) | VC/Q | U-shaped | |
| Marginal cost (MC) | ΔTC/ΔQ | U-shaped; crosses ATC at ATC minimum |
Why cost curves have their shapes
- AFC always declines because a constant is divided by an increasing denominator (spreading the overhead)
- AVC is U-shaped because at low output, adding workers improves efficiency (falling AVC); at high output, crowding and overtime raise costs (rising AVC)
- ATC = AFC + AVC is U-shaped, reaching its minimum at the firm's most efficient scale
- MC is U-shaped because of diminishing returns: holding some inputs fixed, adding more of a variable input eventually produces smaller increments of output, so each additional unit costs more
The MC-ATC relationship
- When MC < ATC, ATC is falling (the next unit pulls the average down)
- When MC > ATC, ATC is rising (the next unit pulls the average up)
- MC crosses ATC at ATC's minimum point
Same relationship holds for MC and AVC.
Short run vs. long run
- Short run: some inputs are fixed (the plant, equipment). The firm can only adjust variable inputs. Fixed costs are sunk in the short run.
- Long run: all inputs are variable. The firm can change everything — size, location, technology, whether to exist at all.
Economies and diseconomies of scale
- Economies of scale: ATC falls as the firm grows (specialization, bulk discounts, spreading fixed costs). Explains why larger firms are often more efficient.
- Diseconomies of scale: ATC rises as the firm gets too large (coordination problems, bureaucracy, communication overhead). Explains why firms don't grow infinitely.
- Minimum efficient scale: the smallest output at which LRATC reaches its minimum.
Decision rules
- Shutdown rule (short run): shut down if P < min AVC (the firm can't even cover variable costs; better to produce nothing and just pay fixed costs)
- Exit rule (long run): exit if P < min ATC (the firm can't cover total costs; resources are better used elsewhere)
- Profit maximization: produce where MR = MC (as long as MR ≥ AVC)
Sunk costs in firm decisions
Sunk costs (already paid, cannot be recovered) should NOT affect forward-looking decisions. The $8M Riverside paid for equipment 10 years ago is irrelevant to today's decision about whether to keep operating. Only forward-looking costs and revenues matter.
The Riverside Foods example
At the current output of 1.2M lbs/month, Riverside earns $165K profit. Expanding to 1.6M lbs would increase profit to $265K (MR > MC for the additional units). Expanding further to 1.8M would reduce profit (MC > MR for those last units). Optimal output: ~1.5–1.6M lbs.
Themes this chapter touched
- Tradeoffs — produce more or less; the MC vs. MR comparison
- Incentives — marginal cost drives the production decision
- Affects daily life — every product you buy reflects a firm's cost structure
One sentence summary
Every cost curve has a shape and the shape has a reason — understanding why marginal cost eventually rises (diminishing returns) and how it relates to average cost (pulling the average up or down) is the foundation of understanding how firms make production decisions.