Short Run vs Long Run Entry/Exit Rules in Perfect Competition

Short Run vs Long Run Entry/Exit Rules in Perfect Competition

Introduction

Understanding how firms decide whether to produce, shut down, enter, or exit a market is crucial in the study of perfect competition. These decisions depend on the relationship between price and various cost measures, and differ between the short run and the long run.

Short Run Decision Rules

In the short run, firms cannot enter or exit the market; they can only decide whether to continue producing or to shut down temporarily. The key cost measure is Average Variable Cost (AVC).

• Produce if P ≥ AVC: The firm covers its variable costs and may contribute to fixed costs.

• Shut down if P < AVC: The firm cannot cover its variable costs and will lose less money by shutting down.

• Profit maximization occurs where P = MC: Firms always maximize profit by producing where price equals marginal cost.

Example: If a firm's AVC is $10 and the market price is $12, the firm should continue producing in the short run.

Key Points

• ATC does NOT matter for short-run shutdown decisions. Only AVC is relevant because fixed costs are sunk in the short run.

• Short-run decision rule: Stay open if P ≥ AVC; shut down if P < AVC.

Long Run Decision Rules

In the long run, all costs are variable and firms can enter or exit the market. The key cost measure is Average Total Cost (ATC).

• Enter if P > ATC: Firms will enter the market if price is above average total cost, as this means positive economic profit.

• Exit if P < ATC: Firms will exit the market if price is below average total cost, as this means economic losses.

• Profit maximization occurs where P = MC: This rule applies in both the short run and long run.

Example: If a firm's ATC is $15 and the market price is $18, new firms will be incentivized to enter the market in the long run.

Key Points

• AVC does NOT matter in the long run. All costs are variable, so ATC is the relevant measure.

• Long-run entry rule: Enter if P > ATC.

• Long-run exit rule: Exit if P < ATC.

Profit Maximization Condition

In both the short run and long run, firms maximize profit where price equals marginal cost.

• Profit maximization: $P = MC$

Summary Table: Entry/Exit and Shutdown Rules

Time Frame	Decision	Rule	Relevant Cost
Short Run	Produce	If $P \geq AVC$	AVC
Short Run	Shut Down	If $P < AVC$	AVC
Long Run	Enter Market	If $P > ATC$	ATC
Long Run	Exit Market	If $P < ATC$	ATC
Always	Profit Maximization	$P = MC$	MC

Long-Run Equilibrium in Perfect Competition

In the long-run equilibrium of a perfectly competitive market:

• Firms earn zero economic profit: $P = ATC$

• Firms produce at the minimum point of ATC: This ensures productive efficiency.

• All three must be equal: $P = MC = ATC$

Example: If $P = MC = ATC = $20, firms are in long-run equilibrium and earn zero economic profit.

Definitions

• Marginal Cost (MC): The additional cost of producing one more unit of output.

• Average Variable Cost (AVC): Variable costs divided by output; relevant for short-run shutdown decisions.

• Average Total Cost (ATC): Total costs divided by output; relevant for long-run entry/exit decisions.

• Economic Profit: Total revenue minus total cost, including both explicit and implicit costs.

Key Equations

• $P = MC$ — Profit maximization

• $P \geq AVC$ — Short-run production

• $P < AVC$ — Short-run shutdown

• $P > ATC$ — Long-run entry

• $P < ATC$ — Long-run exit

• $P = ATC$ — Zero economic profit (long-run equilibrium)

Additional info:

• In the short run, fixed costs are sunk and do not affect shutdown decisions.

• In the long run, all costs are variable, and firms can freely enter or exit the market.

• Perfect competition ensures that resources are allocated efficiently in the long run.