The Definitive Micro Diagram Drill — Wec11
W11-N13 | Version 1 — N-Standard | VERIDIAN™
12 min read
Pearson Edexcel IAL Economics WEC11/01
VERIDIAN™ | © VERIDIAN 2026. All rights reserved. This material is the intellectual property of VERIDIAN. Unauthorised reproduction, resale, or distribution is prohibited. For personal study use only.
Not affiliated with or endorsed by Pearson Edexcel.
PART 1: THE NON-NEGOTIABLE RULES — WEC11 SPECIFIC
RULE 1 — LEVEL 3 REQUIRES A DIAGRAM (confirmed every series)
"NB Level 3 response requires a diagram." — Confirmed in every WEC11 externality, tax, subsidy, and price control mark scheme.
On WEC11, this is absolute. No diagram on a 6-mark, 8-mark, 14-mark, or 20-mark question involving externalities = maximum Level 2 KAA. Draw the diagram BEFORE writing analysis.
RULE 2 — DRAW THE RIGHT DIAGRAM TYPE
"Students were explicitly asked to draw an externalities diagram, so it was a surprise to see so many supply and demand diagrams being drawn." — Jan 2021 Examiner Report
Decision rule:
Question involves EXTERNALITY? → MSC/MPC or MSB/MPB diagram
Question involves TAX/SUBSIDY? → Supply shift diagram
Question involves PRICE CONTROL? → Max/min price diagram
Question involves SUPPLY/DEMAND? → Standard S&D diagram
Never substitute supply/demand for externality diagram. Never use externality diagram where supply shift is needed.
RULE 3 — INTEGRATE THE DIAGRAM IN YOUR WRITING
"When drawing diagrams in essays it is important to be able to access higher marks that it is integrated in the written analysis. For example, by referring to specific areas on the diagram." — Jun 2024 Examiner Report
Write: "As shown in the diagram, the welfare loss triangle ABC..." Not: "The diagram shows the negative externality."
RULE 4 — ZERO TEXT ON DRAW QUESTIONS
"When asked to draw a diagram all marks can be achieved through the diagram and no written explanation is required." — Jun 2022 Examiner Report
Same as WEC12. If asked to DRAW, the diagram earns all marks. Written explanation wastes time and earns zero.
RULE 5 — SPECIFIC VS AD VALOREM TAX SHIFTS
"A £6 per kg tax required a parallel shift in supply." — Jun 2022 Examiner Report (specific tax) "When asked to draw an indirect tax diagram it is important to note whether it is a percentage rate and an ad valorem tax or a set amount." — Jan 2025 Examiner Report
- Specific tax: fixed per unit → supply shifts LEFT by PARALLEL amount (same horizontal distance at every price point)
- Ad valorem tax: percentage of price → supply curve PIVOTS/ROTATES (becomes STEEPER — larger shift at higher prices)
PART 2: ALL EIGHT DIAGRAM TYPES — COMPLETE REFERENCE
DIAGRAM 1 — NEGATIVE EXTERNALITY (MSC > MPC)
When to draw: Any question about negative externalities of production or consumption, overproduction, carbon emissions, pollution.
Y-axis: "Costs and benefits" (exact label from mark schemes) X-axis: "Quantity"
Costs │ MSC
and │ ╱ ╲
benefits │ ╱ ╲ welfare
│ ╱ ▓▓▓▓╲loss
Pso ─┼──────X ╲
Pme ─┼─────╱──────E────╲
│ ╱ │ MPB=MSB
│ ╱ │
└──────────────────────
Qso Qme Quantity
Required elements (K and App marks):
- MSC above MPC (both upward sloping)
- MPB = MSB (downward sloping — labelled)
- Qme: market equilibrium (MPB = MPC intersection) — to the RIGHT (overproduction)
- Qso: social optimum (MSB = MSC intersection) — to the LEFT (socially efficient)
- Pme: market price (lower — under-pricing the externality)
- Pso: social optimum price (higher)
- Welfare loss triangle: shaded area between Qso and Qme (bounded above by MSC, below by MSB)
WRONG version (common error):
- MSC drawn BELOW MPC (backwards — external costs added, not subtracted)
- Qso to the RIGHT of Qme (backwards — overproduction means Qme > Qso)
- Welfare triangle absent or unshaded
DIAGRAM 2 — POSITIVE EXTERNALITY (MSB > MPB)
When to draw: Education, vaccination, healthcare, electric vehicles — underconsumption questions.
Y-axis: "Price" or "Costs and benefits" X-axis: "Quantity"
Price │ S (=MPC)
│ ╱
Pso ─┼─X'────────── (social optimum intersection)
Pme ─┼─E───────────── (market equilibrium)
│╱ ╲ MSB (above MPB)
│ ╲▓▓▓ welfare loss
│ ╲ MPB (market demand)
└──────────────────
Qme Qso Quantity
Required elements:
- MPB: downward sloping (private demand curve)
- MSB: downward sloping, ABOVE MPB at every quantity
- Supply S = MPC: upward sloping
- Qme: market equilibrium (MPB = S) — to the LEFT (underconsumption)
- Qso: social optimum (MSB = S) — to the RIGHT (socially efficient, more)
- Welfare loss triangle: between Qme and Qso
WRONG version (common error):
- MSB drawn BELOW MPB (backwards — external benefits ADD to MPB)
- Qso to the LEFT of Qme (backwards — positive externality means UNDER-production, Qso is higher)
DIAGRAM 3 — SPECIFIC TAX (INDIRECT TAX)
When to draw: Tax on negative externality good, tax incidence questions.
Price │ S+Tax S
│ ╱ ╱ ╱
P1 ──┼─────A ← new equilibrium (tax raises price)
Pe ──┼────E──────────
Pnet ──┼──────B ← producer receives after tax
│ │
│ D │
└──────────────
Q1 Qe Quantity
Required elements:
- Original S and D in equilibrium at Pe, Qe
- S shifts LEFT to S+Tax (specific = parallel shift, same horizontal distance)
- New equilibrium at P1 (higher consumer price), Q1 (lower quantity)
- Pnet: price received by producer (P1 − tax per unit)
- Consumer incidence: rectangle P1 to Pe × Q1
- Producer incidence: rectangle Pe to Pnet × Q1
- Tax revenue: total rectangle P1 to Pnet × Q1 (= consumer + producer incidence)
Key rule: Specific tax = PARALLEL shift. Ad valorem tax = PIVOT (supply curve rotates — becomes steeper).
DIAGRAM 4 — SUBSIDY
When to draw: Government subsidy to correct positive externality, support producers.
Price │ S S+Sub
│ ╱ ╱
P2 ──┼─A─────────── ← producer receives (including subsidy)
Pe ──┼─────E────────────
P1 ──┼──────────B ← new consumer price (lower)
│ │
│ D │
└──────────────────
Qe Q1 Quantity
Required elements:
- S shifts RIGHT to S+Subsidy (parallel shift)
- New equilibrium: lower consumer price P1, higher quantity Q1
- P2: price producer receives (P1 + subsidy per unit) — ABOVE Pe
- Consumer benefit: rectangle Pe to P1 × Q1
- Producer benefit: rectangle Pe to P2 × Q1
- Government cost: rectangle P2 to P1 × Q1 (= consumer + producer benefit)
From Jun 2024 mark scheme: "Consumer surplus increases from PeEF to P1CF; Producer surplus increases from PeEG to P1CH; Government spending ACP1P2."
DIAGRAM 5 — MAXIMUM PRICE (PRICE CEILING)
When to draw: Maximum rent, food price ceiling — set BELOW equilibrium.
Price │ S
│ ╱ |
Pe ──┼───E | ← free market equilibrium (above ceiling)
│ ╱ |
Pmax ──┼──────|────────← price ceiling (below Pe)
│ Qs| Qd D
└──────────────
Qs Qd Quantity
shortage = Qd - Qs
Required elements:
- Original equilibrium at Pe, Qe
- Horizontal price ceiling line at Pmax < Pe
- At Pmax: Qd > Qe (demand extends — consumers want more at lower price)
- At Pmax: Qs < Qe (supply contracts — producers supply less at lower price)
- Shortage clearly labelled: Qd - Qs = excess demand
WRONG version: Price ceiling set ABOVE Pe has no effect (price won't rise above Pe anyway). Ceiling must be BELOW equilibrium to matter.
DIAGRAM 6 — MINIMUM PRICE (PRICE FLOOR / GUARANTEED PRICE)
When to draw: Agricultural minimum prices, minimum wage, minimum alcohol price — set ABOVE equilibrium.
Price │ S
Pmin ──┼──────────────← minimum price (above Pe)
Pe ──┼───E ← free market equilibrium (below floor)
│ ╱ ╲
│ D╲
└──────────────
Qd Qs Quantity
surplus = Qs - Qd
Required elements:
- Original equilibrium at Pe, Qe
- Horizontal minimum price line at Pmin > Pe
- At Pmin: Qs > Qe (supply extends — producers supply more at higher price)
- At Pmin: Qd < Qe (demand contracts — consumers buy less at higher price)
- Surplus clearly labelled: Qs - Qd = excess supply
- Government must PURCHASE surplus to maintain Pmin (fiscal cost shown or noted)
DIAGRAM 7 — CONSUMER AND PRODUCER SURPLUS
When to draw: Questions specifically asking about CS/PS, welfare effects of policy.
Price │ D
│╲
│ ╲ CS (area above Pe, below D)
Pe ──┼──E────────── (equilibrium)
│ ╱ PS (area below Pe, above S)
│ ╱
│╱ S
└──────────────
Qe Quantity
CS = triangle: Bounded above by demand curve, below by Pe, from 0 to Qe. PS = triangle: Bounded below by supply curve, above by Pe, from 0 to Qe. Total welfare = CS + PS (maximised at competitive equilibrium)
DIAGRAM 8 — SUPPLY AND DEMAND EQUILIBRIUM
When to draw: Price determination, market changes, shifts in S or D — standard micro question.
Y-axis: "Price" X-axis: "Quantity"
Rules identical to WEC12 but for individual market (not aggregate economy):
- Dotted lines from equilibrium to BOTH axes
- Label equilibrium: P₁, Q₁
- After shift: new equilibrium P₂, Q₂ with dotted lines to both axes
- Arrow on shifted curve
PART 3: SAME DIAGRAM AT THREE LEVELS — NEGATIVE EXTERNALITY
Question: "Draw a diagram to illustrate the market failure resulting from negative externalities of production."
LEVEL 2 (2/4): Student draws a supply and demand diagram with an upward sloping supply curve and downward sloping demand curve. Labels P and Q at equilibrium.
What's wrong: Wrong diagram type — supply/demand not MSC/MPC. No external cost represented. No social optimum. No welfare loss triangle.
LEVEL 3 (3/4): Student draws MSC above MPC (correct direction). Demand curve labelled MPB = MSB. Identifies Qme at MPB = MPC intersection and Qso at MPB = MSC intersection. Qso correctly shown to LEFT of Qme. Does NOT shade welfare loss triangle.
What's missing: Welfare loss triangle unshaded and unlabelled. Mark scheme confirms the triangle identifies the deadweight loss — its absence means the answer doesn't show the full extent of the market failure.
LEVEL 4 (4/4): As Level 3 PLUS:
- Welfare loss triangle explicitly shaded between Qso and Qme
- Welfare loss triangle labelled (can write "welfare loss" inside the triangle or point to it)
- Both Pme and Pso labelled with dotted lines to Y-axis
- Zero written text on the diagram itself
PART 4: WRONG VS RIGHT — THREE CRITICAL ERRORS
ERROR 1 — WRONG DIAGRAM TYPE (externality question, supply/demand drawn)
WRONG (supply/demand for externality): CORRECT (MSC/MPC diagram):
Price │ S Costs │ MSC
│ ╱ and │ ╱ ╲
Pe ──┼─E──── bens │ ╱ ▓▓╲
│╱ ╲ │ ╱ ╲
│ D Pso ──┼────X ╲
└───────── Pme ──┼───╱─────E──── MPB
Qe └──────────────
Qso Qme
Cost: Level 2 KAA maximum immediately. Diagram earns no K marks. Fix: identify "externality question" → draw MSC/MPC, not S/D.
ERROR 2 — MSB BELOW MPB (backwards positive externality)
WRONG (MSB below MPB): CORRECT (MSB above MPB):
Price │ S Price │ S
│ │
Pe ─┼──E Pe ─┼──E
│ ╲ MPB │ ╲ ─── MSB (ABOVE)
│ ╲──────── MSB (wrong!) │ ╲ ╲
└────────── └──────── ─╲MPB
Qme Qme Qso
(no underconsumption shown) (underconsumption shown ✓)
Fix: External benefits ADD to private benefits. MSB = MPB + external benefit. MSB is always ABOVE MPB.
ERROR 3 — AD VALOREM TAX AS PARALLEL SHIFT
WRONG (ad valorem as parallel): CORRECT (ad valorem as pivot):
Price │ S+Tax S Price │ S+Tax S
│ ╱ ╱ │ ╱ ╱
│╱ ╱ ← same distance │ ╱ ╱ ← gap widens
│ ╱ at all prices │╱╱ at higher prices
└────── └──────
Ad valorem tax is a percentage — at higher prices, the same percentage = a larger absolute amount. The gap between S and S+Tax widens as price increases. The supply curve PIVOTS (rotates) rather than shifting in parallel.
PART 5: DIAGNOSE YOUR DIAGRAM — FIVE STUDENT DESCRIPTIONS
DESCRIPTION 1: "I drew two axes. Y-axis: 'Price'. X-axis: 'Quantity'. Drew a downward sloping demand curve and an upward sloping supply curve. For the negative externality, I drew another supply curve to the right to show external costs."
Level: 0–1/4. Wrong diagram type (S/D not MSC/MPC). Supply shifting RIGHT shows a supply increase — the opposite of an external cost. External costs ADD to private costs → MSC is ABOVE MPC (not shifted right). Fix: draw MSC ABOVE MPC, identify Qme to right of Qso, shade welfare loss triangle.
DESCRIPTION 2: "I drew MSC above MPC (both upward sloping) and MPB as downward sloping. I found where MPB meets MPC (market equilibrium, labelled Qme). I found where MPB meets MSC (social optimum, labelled Qso). I put Qme to the right of Qso. I labelled the prices Pme and Pso."
Level: 3/4. Correct diagram type ✓. Direction correct (Qme right of Qso) ✓. Prices labelled ✓. Missing: welfare loss triangle shaded and labelled. Fix: shade the triangular area between Qso and Qme, bounded above by MSC and below by MSB. Label "welfare loss triangle." +1 mark.
DESCRIPTION 3: "For the subsidy diagram I drew supply shifting to the RIGHT. New equilibrium: lower price P1, higher quantity Q1. I labelled the price producers receive as P2 (P1 + subsidy). I shaded the areas for consumer benefit and producer benefit."
Level: 4/4. All elements present. Direction correct (S shifts right for subsidy ✓). Price levels correct ✓. Consumer and producer incidence shaded ✓. This is Level 4.
DESCRIPTION 4: "For the maximum price diagram I drew a horizontal line at Pmax below the equilibrium Pe. At Pmax, I identified that more was demanded (Qd) and less was supplied (Qs). I labelled the shortage as Qd - Qs."
Level: 4/4. All elements correct. Pmax below Pe ✓. Qd > Qe ✓. Qs < Qe ✓. Shortage labelled ✓. This is Level 4.
DESCRIPTION 5: "I drew a positive externality diagram. MSB is below MPB because positive externalities reduce the social benefit below the private benefit."
Level: 0/4. Fundamental error. Positive externalities ADD to private benefits — MSB is ABOVE MPB, not below. Fix: understand that external benefits are additional benefits enjoyed by third parties. MSB = MPB + external benefit → MSB must be above MPB.
PART 6: 60-SECOND DIAGRAM CHECKLIST
Before every diagram question:
□ What type is it? Externality / Tax / Subsidy / Price control / S&D?
□ Drawn the correct diagram type?
□ Y-axis: "Costs and benefits" (externality) or "Price" (S/D, tax, subsidy)?
□ X-axis: "Quantity"?
□ All curves labelled (MSC, MPC, MPB, MSB / S, D / S+Tax)?
□ Equilibrium points labelled with dotted lines to BOTH axes?
□ Welfare loss triangle SHADED (externality)?
□ Surplus/shortage clearly labelled (price controls)?
□ ZERO written text on the diagram?
All checked = 4/4. Time: 4 minutes.
VERIDIAN™ | © VERIDIAN 2026. All rights reserved. This material is the intellectual property of VERIDIAN. Unauthorised reproduction, resale, or distribution is prohibited. For personal study use only. Not affiliated with or endorsed by Pearson Edexcel.
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