Firms decide whether to operate in the short run by comparing price to average variable cost (AVC), and they decide whether to enter or exit a market in the long run based on economic profit or loss. If price falls below AVC, a firm shuts down in the short run; if a firm faces ongoing economic losses and there are no barriers to entry or exit, it leaves the market in the long run.
AP Micro 3.6 Summary
For AP Micro Topic 3.6, the short-run decision is about whether a firm should produce a positive output or shut down. The rule is direct: produce if price is at least AVC, or equivalently if total revenue is at least total variable cost. Shut down if price is below AVC.
The long-run decision is different because all costs become variable. If firms see profit-making opportunities and there are no barriers to entry, firms enter. If firms anticipate economic losses, firms exit. Entry pushes market supply right and price down; exit pushes market supply left and price up.
Why This Matters for the AP Microeconomics Exam
This topic connects profit maximization to real firm decisions, which is exactly the kind of reasoning the AP Microeconomics exam tests. You should be able to read a firm graph or a cost table and explain whether a firm should keep producing or shut down, then predict whether firms enter or exit the industry over time. These cause-and-effect chains show up in both multiple-choice questions and free-response prompts that ask you to identify decisions and explain the adjustment process toward long-run equilibrium.
The decisions here feed directly into the perfect competition model in Topic 3.7, so getting the short-run shutdown logic and the long-run entry/exit logic right now makes that later material much easier.
Key Takeaways
- Short-run decision: compare price to AVC, or total revenue to total variable cost. Operate if price is at or above AVC; shut down if price falls below AVC.
- A shut-down firm still pays fixed costs, so the goal is to minimize losses, not necessarily to avoid all loss.
- Long-run decision: with no barriers to entry or exit, firms enter when there are profit-making opportunities and exit when they expect economic losses.
- In the long run all costs are variable, so an exiting firm can avoid all costs, unlike in the short run.
- Entry shifts market supply right and pushes price down; exit shifts market supply left and pushes price up.
- Because firms are price takers, this entry and exit process drives economic profit toward zero (normal profit) in the long run.
Entering and Exiting a Market
In a perfectly competitive market there are many sellers and low barriers to entry. In theory, anyone can open a business in that market, and anyone can leave it. This topic explains how and why a firm chooses to produce, shut down, enter, or exit, but first a few definitions.
A firm enters a market when it begins selling output, meaning its quantity is greater than zero. A firm exiting a market is not the same as the business completely disappearing. Exiting means the firm has decided to produce zero output in that market. For the purposes of this model, exiting means producing nothing.
The Shut-Down Rule
In the short run, firms decide to operate or shut down by comparing total revenue to total variable cost, or price to average variable cost (AVC). Fixed costs exist no matter what in the short run, even at zero output, so they do not affect the operate-versus-shutdown choice.
The shutdown rule: a firm should keep operating as long as price is at or above AVC. If price falls below AVC, the firm shuts down and produces zero.
In the graph above, at the profit-maximizing quantity (where MR = MC) of 10, the price is below the AVC of $7. Based on the shutdown rule, this firm chooses not to operate and instead pays only its fixed costs, because that loss is smaller than the loss from continuing to produce.
The core idea: if you cannot cover the variable cost of producing, then producing makes your loss worse. Shutting down limits the loss to fixed costs.
Look at the specific numbers. Total revenue at the profit-maximizing point is 5 x 10). Total cost is 12 x 10). Operating produces an economic loss of 50 - $120). If the firm shuts down instead, it only owes its fixed costs.
The vertical difference between ATC and AVC is average fixed cost (AFC), which here is 12 - $7). Multiply AFC by quantity (10) to get total fixed cost of $50. Shutting down and losing $50 beats operating and losing $70.
Out of scope for AP Micro, but worth knowing: producer surplus for a firm equals its variable profit, so PS = TR - VC (not TC). When price is below variable cost in a perfectly competitive market, the firm has negative producer surplus. That is part of the reasoning behind the shut-down rule.
Long-Run Decisions to Enter or Exit the Market
When a firm earns a profit or a loss in the short run, that result signals to other firms whether to enter or exit the market. If a firm keeps operating at a loss in the short run, firms will start leaving that industry in the long run. In the long run all inputs are variable, so a firm can avoid all of its costs by exiting (no costs stay fixed in the long run). Firms keep exiting until the remaining firms earn at least normal profit, at which point entry can occur again.
In the graph below, a firm is earning a loss in the short run but is still operating. Over time, individual firms leave the industry because there is no profit available.
Here total revenue is 5) and total cost is 12), an economic loss of $70. Fixed costs are $90 (the vertical difference between ATC and AVC, $9, times the quantity, 10). Because fixed costs are greater than the economic loss, and AVC is below price, the firm keeps operating in the short run rather than shutting down. Even so, firms leave this industry over time because there is no potential for profit.
When a firm earns a profit in the short run, new firms are attracted to the industry. In the graph below, total revenue is $50 and total cost is only $30, so the firm earns an economic profit of $20. That profit makes other firms interested in entering.
What Happens to the Market When Firms Enter or Exit?
Because perfectly competitive firms are price takers, the market price moves as firms enter and exit. You can see this by viewing the market and the firm side by side.
Price is set by the market equilibrium, so any shift in supply or demand changes the firm's horizontal demand line, where MR = D = AR = P.
- When a firm enters, market supply shifts right, which lowers the price and pushes MR = D = AR = P down.
- When a firm exits, market supply shifts left, which raises the price and pushes MR = D = AR = P up.
Because firms enter only when there are profits and leave only when there are losses, economic profit moves toward zero (normal profit) in the long run. If profits exist, entry drives price down until profit reaches zero. If losses exist, exit drives price up until losses disappear. Topic 3.7 builds on this adjustment process in more detail.
How to Use This on the AP Microeconomics Exam
Problem Solving
- Find the profit-maximizing quantity first, where MR = MC. Then check price against AVC at that quantity to decide operate versus shut down.
- To compare losses, calculate the loss from operating (TR - TC) and the loss from shutting down (total fixed cost). The firm picks the smaller loss.
- Use the gap between ATC and AVC to find AFC, then multiply by quantity to get total fixed cost.
Free Response
- State the short-run decision clearly (operate or shut down) and justify it with the price-versus-AVC comparison, not just "it is losing money."
- For long-run prompts, explain the direction of entry or exit, the resulting shift in market supply, and the effect on price until economic profit returns to zero.
- Connect each step with cause and effect: profit attracts entry, entry shifts supply right, price falls, profit shrinks toward zero.
Common Trap
- A short-run loss does not automatically mean shut down. A firm operates at a loss as long as price is at or above AVC, because it is still covering variable costs and part of fixed costs.
Common Misconceptions
- Exiting a market is not the same as a firm going bankrupt or disappearing. In this model, exit just means producing zero output in that market.
- Shutting down does not make losses go to zero. A shut-down firm still owes fixed costs in the short run, so it minimizes loss rather than eliminating it.
- The shutdown decision uses AVC, not ATC. Comparing price to ATC tells you about profit or loss, but the operate-versus-shutdown choice depends on AVC.
- Fixed costs do not belong in the short-run operate-versus-shutdown comparison, because they are paid whether or not the firm produces.
- Entry and exit happen in the long run, not instantly. Short-run profits and losses are the signal; the market adjusts over time as supply shifts.
- "Zero economic profit" in long-run equilibrium does not mean the firm earns nothing. It means the firm earns normal profit, covering all costs including implicit costs.
Related AP Microeconomics Guides
Vocabulary
The following words are mentioned explicitly in the College Board Course and Exam Description for this topic.Term | Definition |
|---|---|
average variable cost | The total variable cost divided by the quantity of output produced; used to determine whether a firm should operate or shut down in the short run. |
barriers to entry | Obstacles that prevent new firms from entering a market, allowing existing firms to maintain market power. |
barriers to exit | Obstacles that prevent firms from leaving a market, such as long-term contracts or sunk costs. |
economic losses | A situation where a firm's total revenue is less than its total economic cost, resulting in negative economic profit. |
long run | A time period in which all factors of production are variable, allowing firms to enter or exit markets and adjust all inputs. |
profit-making opportunities | Situations in which firms can earn economic profits by entering a market or continuing operations. |
short run | A time period in which at least one factor of production is fixed, and firms can only adjust variable inputs to change output levels. |
shut down | A short-run decision by a firm to produce zero output when price falls below average variable cost. |
total revenue | The total income a firm receives from selling its goods or services, calculated as price multiplied by quantity sold. |
total variable cost | The sum of all costs that vary with the level of output produced in the short run. |
Frequently Asked Questions
When will a firm shut down in the short run?
A firm shuts down in the short run when price is below average variable cost, or when total revenue is less than total variable cost. Producing would make the loss larger than paying fixed costs only.
What is the short-run shutdown rule in AP Micro?
The shutdown rule is: produce if P is at least AVC, and shut down if P is less than AVC. The same idea can be written as produce if TR is at least TVC.
Why can a firm operate at a loss in the short run?
A firm can operate at a loss if price covers average variable cost and contributes something toward fixed cost. Shutting down would still require paying fixed costs, so operating can be the smaller loss.
When do firms enter a market in the long run?
Firms enter in the long run when there are profit-making opportunities and no barriers to entry. Entry increases market supply and pushes price downward.
When do firms exit a market in the long run?
Firms exit in the long run when they anticipate economic losses and there are no barriers to exit. Exit decreases market supply and pushes price upward.
What is a common mistake on AP Micro 3.6?
A common mistake is using ATC for the shutdown decision. ATC tells you profit or loss, but AVC determines whether the firm produces or shuts down in the short run.