Graphs to Know for AP Microeconomics (AP)

Map of the Must-Know Graphs

You’re not just memorizing pictures—you’re mastering relationships (what changes, what shifts, what moves along, and where efficiency lives). AP Micro loves asking you to:

• Find equilibrium and identify shortage/surplus
• Track policy changes (taxes, subsidies, price controls)
• Measure surplus, deadweight loss, and welfare changes
• Do firm decisions with cost curves and market structures
• Connect product markets to factor markets (labor)

Core rule: most AP Micro graphs boil down to where two curves intersect and what happens when a curve shifts.

Critical reminder: A shift changes the whole curve (new curve). A movement along a curve happens when the other axis variable changes (like price causing movement along demand).

1. What You Need to Know

The big graph families (what they represent)

1. Market (Supply & Demand) graphs: how buyers/sellers interact to set $P$ and $Q$.
2. Welfare graphs: consumer surplus, producer surplus, taxes, deadweight loss.
3. Firm cost & revenue graphs: how firms choose output using $MC$, $MR$, and cost curves.
4. Market structure graphs: perfect competition vs monopoly (and the “in-between” structures).
5. Externality graphs: when private markets misallocate resources (MSC/MSB vs MPC/MPB).
6. Factor market (labor) graphs: hiring decisions using $MRP$ and $MFC$.

The single most-tested decision rule

For output choice (firm side):

• Profit-maximizing quantity is where $MR=MC$ (as long as $MR$ crosses $MC$ from above).
• Profit is $\pi=TR-TC$.

For market equilibrium (market side):

• Equilibrium is where $Q_D=Q_S$.

2. Step-by-Step Breakdown

A. How to attack any Supply–Demand policy graph (tax, subsidy, ceiling, floor)

1. Draw baseline supply $S$ and demand $D$; mark equilibrium $\left(P^*,Q^*\right)$.
2. Identify which curve shifts (or if it’s a control line):
   • Tax on sellers: shifts supply up/left by amount of tax.
   • Tax on buyers: shifts demand down/left by amount of tax.
   • Subsidy: opposite direction.
   • Price ceiling/floor: draw a horizontal line at the policy price.
3. Find the new transacted quantity:
   • With taxes/subsidies: new intersection gives $Q_{new}$.
   • With price controls: quantity is the smaller of $Q_D$ and $Q_S$ at that price.
4. Compute incidence/welfare regions:
   • Tax wedge: buyers pay $P_b$, sellers receive $P_s$, and $P_b-P_s=t$.
   • Tax revenue: rectangle $t\times Q_{tax}$.
   • Deadweight loss: triangle between $D$ and $S$ over the units not traded.

B. How to do firm profit on cost curves

1. Choose $Q^*$ where $MR=MC$.
2. Read price:
   • Perfect competition: $P=MR=AR=D_{firm}$.
   • Monopoly: price comes from demand at $Q^*$.
3. Find $ATC$ at $Q^*$.
4. Profit rectangle:
   • Profit if $P>ATC$: $\pi=(P-ATC)\times Q^*$.
   • Loss if $P<ATC$.
5. Shutdown check (short run):
   • Produce if $P\ge AVC$.
   • Shutdown if $P<AVC$.

C. How to do externalities (efficient output)

1. Draw MPB (demand) and MPC (supply).
2. Add external curves:
   • Negative externality: $MSC$ lies **above** $MPC$.
   • Positive externality: $MSB$ lies **above** $MPB$.
3. Market outcome is MPB=MPC.
4. Socially efficient outcome is MSB=MSC.
5. DWL is triangle between social and private curves over the misproduced units.

3. Key Formulas, Rules & Facts

Master graph reference table

High-yield rules (graph behavior)

4. Examples & Applications

Example 1: Per-unit tax (welfare + incidence)

Setup: Market for coffee. Government imposes $t$ per unit tax.

• Draw $D$ and $S$, then draw shifted supply $S_{tax}$ **up** by $t$.
• New intersection gives $Q_{tax}$.
• Buyers pay $P_b$ (read off demand at $Q_{tax}$), sellers receive $P_s$, with $P_b-P_s=t$.
  Key insight:
• Tax revenue is rectangle $t\times Q_{tax}$.
• DWL is triangle between $D$ and $S$ over $Q^*-Q_{tax}$.
• More inelastic side bears more burden (bigger price change on that side).

Example 2: Perfectly competitive firm (shutdown vs produce)

Setup: A firm in perfect competition faces market price $P$.

• Draw horizontal $P=MR=AR$ line.
• Choose $Q^*$ where $P=MC$.
• If at $Q^*$ you see $P<AVC$, the firm shuts down (produces $Q=0$).
  Key insight:
• AP loves the distinction between operating loss (produce with $P<ATC$ but $P\ge AVC$) vs **shutdown** (when $P<AVC$).

Example 3: Monopoly (profit + DWL)

Setup: Monopolist with demand $D$ and marginal revenue $MR$.

• Pick $Q^*$ where $MR=MC$.
• Move up to demand to get $P_m$.
• Profit rectangle is $\left(P_m-ATC\right)\times Q^*$.
  Key insight:
• Deadweight loss: compare to competitive outcome where $P=MC$ (or where $D$ intersects $MC$). Units between $Q_m$ and $Q_c$ that would create net benefits are not produced.

Example 4: Negative externality (efficient output)

Setup: Pollution from production.

• Market outcome at $MPB=MPC$ gives $Q_{market}$.
• Social optimum at $MSB=MSC$ gives lower output $Q_{social}$.
  Key insight:
• Corrective tax equal to marginal external cost at $Q_{social}$ shifts $MPC$ up to $MSC$ and moves market to efficient outcome.

5. Common Mistakes & Traps

1. Mixing up shifts vs movements: You shift demand/supply for non-price determinants; you move along for price changes. Fix: ask “Did the variable change on an axis?” If yes, movement along.
2. Putting tax incidence on the wrong prices: Students label $P_b$ and $P_s$ backwards. Fix: buyers’ price is on the demand curve at the traded quantity; sellers’ price is on the original supply curve at that quantity.
3. Using the wrong quantity under price controls: Students use $Q_D$ or $Q_S$ incorrectly. Fix: **quantity actually exchanged** is the **smaller** of $Q_D$ and $Q_S$ at the controlled price.
4. Forgetting the shutdown condition: Students shut down whenever $P<ATC$. Fix: shutdown is $P<AVC$; if $AVC\le P<ATC$, produce to cover variable costs.
5. Monopoly: setting price where $MR=MC$: That point gives quantity, not price. Fix: choose $Q$ at $MR=MC$, then go up to demand for price.
6. Mislabeling $MC$ intersections: Students draw $MC$ crossing $ATC$/$AVC$ off-center. Fix: $MC$ always crosses the **minimum** of $AVC$ and $ATC$.
7. Externality graph confusion (MSC/MSB directions): Students put $MSC$ below $MPC$ for a negative externality. Fix: negative externality means higher true marginal cost, so $MSC$ is **above** $MPC$.
8. Calling monopoly DWL the whole triangle under demand: Only the triangle between competitive and monopoly quantities (bounded by demand and $MC$) is DWL.

Warning: If you can’t clearly say what each curve means (not just its shape), you’ll miss the FRQ even with a decent sketch.

6. Memory Aids & Quick Tricks

7. Quick Review Checklist

• Can you draw and label S&D with equilibrium $\left(P^*,Q^*\right)$ and show shortage/surplus?
• Can you find CS, PS, tax revenue, and DWL regions correctly?
• For price ceilings/floors, do you use the smaller of $Q_D$ and $Q_S$ as the traded quantity?
• For a per-unit tax, can you show the wedge $P_b-P_s=t$?
• For firm graphs, can you label $MC, ATC, AVC$ and apply $MR=MC$?
• In perfect competition, do you remember $P=MR=AR$ and shutdown $P<AVC$?
• In monopoly, do you pick $Q$ at $MR=MC$ and then read $P$ from demand?
• For externalities, can you show market $MPB=MPC$ vs **efficient** $MSB=MSC$ and the DWL triangle?
• For labor markets, can you use $MRP$ as labor demand and distinguish competitive hiring vs monopsony?

You’ve got this—if you can sketch these cleanly and narrate what shifts and why, you’re in great shape for the AP Micro exam.