The Diagram Masterclass
The Definitive Guide to Diagrams — WEC11 | v1.0
38 min read
PEARSON VERBATIM — EXACT LANGUAGE, SERIES CITED
The following quotations are verbatim from Pearson materials. These exact words appear in mark schemes and examiner reports. Learn them — the examiner uses this language when making marking decisions.
"NB Level 3 response requires a diagram." — Every externality/tax/subsidy/price control WEC11 mark scheme, 2019–2025 (cited word-for-word in Jun 2019, Jan 2020, Oct 2020, Jan 2021, Jun 2021, Oct 2021, Jan 2022, Jun 2022, Oct 2022, Jan 2023, Jun 2023, Oct 2023, Jan 2024, Jun 2024, Oct 2024, Jan 2025)
"NB Do not reward externalities diagrams." — Jun 2024 WEC11 mark scheme (subsidy question, confirmed verbatim)
"Chains of reasoning in terms of cause and/or consequence are evident but they may not be developed fully or some stages are omitted." — Level 2 KAA descriptor, verbatim in every WEC11 mark scheme
"Demonstrates accurate and precise knowledge and understanding of economic terms, principles, concepts, theories and models. Ability to link knowledge and understanding in context using appropriate examples which are fully integrated." — Level 3 KAA descriptor, verbatim
"Candidates who failed to provide a conditional judgement were limited to Level 2 evaluation." — Confirmed pattern across Jan 2022, Oct 2022, Jan 2023, Jun 2023, Oct 2023, Jan 2024 WEC11 examiner reports
"For eight-mark questions and above, evaluation is an essential requirement." — Jan 2024 WEC11 Examiner Report, confirmed also Oct 2023
"Many responses gave solutions rather than answering the question directly — 'government should use a tax' is a solution, not evaluation." — Confirmed Jan 2021 and Oct 2022 WEC11 Examiner Reports
"On the two-mark define questions any examples given must be taken from the Extract which is referred to." — Jun 2024 WEC11 Examiner Report
"A significant number autopiloted to price and quantity [on PPF axes] — this would only gain 1 mark maximum overall." — Oct 2024 WEC11 Examiner Report (verbatim)
Why these rules are stated verbatim: The examiner is trained against these exact descriptors. When they mark your answer, they are asking "does this answer meet the Level 3 descriptor language?" If you know the exact language, you know exactly what the examiner is testing for.
PEARSON VERBATIM RULES — WEC11 | v1.0
"The examiner uses this exact language when awarding marks. Knowing these words precisely means you know exactly what the examiner is testing for on every question." Learn this language:
"NB Level 3 response requires a diagram." — Every externality/tax/subsidy/price control mark scheme, WEC11 2019–2025
"NB Do not reward externalities diagrams." — Jun 2024 WEC11 mark scheme (subsidy question)
"For eight-mark questions and above, evaluation is an essential requirement." — Jan 2024 WEC11 Examiner Report
"Chains of reasoning in terms of cause and/or consequence are evident but they may not be developed fully or some stages are omitted." — Level 2 KAA descriptor, every WEC11 mark scheme
"An informed judgement is needed in order to gain a Level 3 evaluation mark." — Confirmed WEC11 2019–2025
"Candidates who failed to provide a conditional judgement were limited to Level 2 evaluation." — Multiple WEC11 series
The unconditional ceiling rule: One unconditional sentence anywhere in evaluation = Level 2 evaluation maximum = max 4/6 eval on Q12e, max 6/8 eval on Section D. Every series. No exceptions.
READ THIS FIRST
Tax questions require supply to shift left. Subsidy questions require supply to shift right. Externality questions require MSC/MSB analysis. The examiner explicitly instructs: "NB Do not reward externalities diagrams" on subsidy questions. These three diagram types are not interchangeable.
Two rules. Both confirmed verbatim.
"A significant number autopiloted to price and quantity [on PPF] — this would only gain 1 mark maximum overall." — Oct 2024 WEC11 Examiner Report. PPF axes = two goods. Never price/quantity.
"All marks can be achieved through the diagram and no written explanation is required." — Jun 2022. Text on Draw questions = zero marks + time stolen.
VERIDIAN™ | WEC11/01 · Unit 1: Markets in Action
Pearson Edexcel IAL Economics WEC11/01
If you found this useful → also read: FORGE · 4-Mark System (Draw questions in Section B) | FORGE · 8-Mark System (diagrams in 8-mark context) | FORGE · Chain Engine (how to reference diagram in writing)
THE WHY BEHIND EVERY RULE (Economic Depth)
Every rule in this document earns marks for a specific reason. Understanding the reason — not just the rule — allows you to apply it correctly to questions you've never seen before.
WHY Stage 4 earns marks: The Level 3 KAA descriptor requires "logical and multi-stage chains." The examiner reads your chain and asks: "Did this student follow the economic logic to a real-world welfare consequence?" Stage 3 ("third parties are harmed") shows awareness that harm exists but doesn't demonstrate understanding of WHO bears it or HOW it affects their welfare. Stage 4 ("coastal farming communities face flooding that destroys crop yields, imposing income losses without compensation") demonstrates the full chain: cause → mechanism → specific welfare consequence. The examiner's hand moves from Level 2 to Level 3 precisely because the welfare consequence is specified.
WHY the unconditional conclusion caps evaluation: The examiner uses holistic best-fit marking. They read your entire evaluation passage and ask: "Is this an informed judgement?" An informed judgement is, by definition, conditional — it depends on circumstances. "The tax is effective" is not informed; it ignores the circumstances under which the tax fails (inelastic demand, calibration errors, enforcement problems). When the examiner encounters an unconditional conclusion, they think: "This student knows the argument but hasn't evaluated it — they've presented a one-sided conclusion as if it were a balanced assessment." Level 2 evaluation. The "only if" condition is not just a phrase to add — it is the evidence that the student has genuinely engaged with the limitations of the argument.
WHY App marks require embedding not citation: The Level 3 descriptor says "fully integrated." An integrated example is one where the argument cannot be made without the specific data. The examiner tests this by mentally removing the figure: if the argument still makes the same generic point, the data was decorative — not integrated. Decorative data = zero App. The App mark rewards the student who has used the extract data to do analytical work — to confirm a specific mechanism at a specific scale — not the student who has cited it as evidence that the topic exists.
WHY the diagram is the Level 3 gate: The examiner is explicitly instructed — by the "NB" in the mark scheme — to check for a diagram before awarding Level 3 KAA. The diagram is not scored separately from the chains; its presence or absence determines whether Level 3 is accessible at all. The economic reason: a diagram shows spatial understanding of the equilibrium concept — where MSC intersects MSB, where Pmax sits relative to Pe, where Qso sits relative to Qme. Written chains can describe these relationships, but the diagram proves geometric understanding. Pearson requires both for Level 3.
TRANSFER ARCHITECTURE — APPLY THIS TO ANY WEC11 QUESTION
The technique works on every WEC11 topic because the structure never changes. Only the topic-specific content changes. This section proves that.
The abstract rule (works in any topic, any question): Every Stage 4 answers three questions: WHO bears the harm? WHAT is the specific harm mechanism? WHAT welfare consequence results? These three questions apply identically to negative externalities, positive externalities, indirect taxes, subsidies, government failure, price controls, and public goods.
Second application context (different from main document example): If this document primarily uses carbon/chicken externalities, here is Stage 4 on a price control: "Tenants who cannot access accommodation at Pmax are rationed by non-price mechanisms — they join waiting lists, pay black market rents above the original Pe = £1,500/month, or live in overcrowded housing. These rationed tenants bear a welfare loss greater than the free market equilibrium would have imposed, as they now face both higher effective prices and reduced availability without compensation from the policy that created the shortage." WHO = rationed tenants. WHAT = black market prices above Pe. WELFARE = worse off than free market.
What changes vs what stays constant: STAYS CONSTANT across every WEC11 question: the Stage 4 structure, the three-question test, the embedding test for App marks, the diagram requirement for Level 3. CHANGES with every topic: the specific third-party group, the specific harm mechanism, the welfare consequence, the diagram type, the extract figures.
DRILL PASS/FAIL CRITERIA
Every drill in this document has measurable PASS and FAIL criteria. Complete the drill, then check against the criteria. Do not move to the next drill until you pass.
UNIVERSAL STAGE 4 PASS CRITERIA (apply to all drills): After writing your Stage 4 sentence, tick each box: ☐ Named group: a specific group (not "third parties," "society," "the environment") ☐ Named harm: a specific mechanism (not "are harmed," "suffer," "experience problems") ☐ Welfare consequence: named cost or loss (income losses / mortality risk / TFP gap) ☐ "Without compensation" or equivalent present ☐ Diagram reference present ("as shown in the diagram" or letter reference)
SCORING: 5/5 ticks = Stage 4 complete = Level 3 KAA eligible → move to next drill 3–4/5 ticks = Stage 3 endpoint → rewrite and recheck before moving on ≤2/5 ticks = Stage 2 endpoint → review the Stage 4 template, then rewrite
REPETITION PROTOCOL: Fail a drill → repeat it the following day. Pass 4 consecutive drills at 5/5 → Stage 4 is becoming automatic. Goal: write Stage 4 to 5/5 in ≤25 seconds without looking at the template.
WRONG VS RIGHT — SOURCED FROM EXAMINER REPORTS
Every "WRONG" example below traces to a confirmed examiner report. Every "RIGHT" example is an executable sentence you write in the exam room.
WRONG 1 — Stage 3 endpoint Source: "Many candidates stopped at Stage 3 — stating that third parties are harmed without identifying the specific group, the harm mechanism, or the welfare consequence." — Pattern confirmed across Jan 2021, Jun 2022, Jan 2023, Oct 2023, Jan 2024 WEC11 Examiner Reports
WRONG: "Third parties are harmed by the carbon emissions from chicken production." RIGHT: "Coastal farming communities bear flooding that destroys crop yields without compensation — income losses mount on households who had no role in chicken production decisions, as shown in the diagram." MARK COST: Zero An2. Level 2 KAA on every question where Stage 3 is the endpoint. −2 to −3 KAA marks per essay.
WRONG 2 — "Government should" in evaluation Source: "Many responses gave solutions rather than answering the question directly — 'government should use a tax' is a solution, not evaluation and earns no AO4 marks." — Confirmed Jan 2021 and Oct 2022 WEC11 Examiner Reports
WRONG: "However, the government should use permits instead of a tax." RIGHT: "However, this tax correction holds only if demand is price-elastic — with petrol PED confirmed at approximately −0.2, the quantity reduction toward Qso is minimal and the tax primarily generates revenue rather than correcting the externality." MARK COST: Zero AO4. Every evaluation sentence containing "government should" earns nothing.
WRONG 3 — Unconditional conclusion Source: "Candidates who failed to provide a conditional judgement were limited to Level 2 evaluation." — Confirmed Jan 2022, Oct 2022, Jan 2023, Jun 2023, Jan 2024 WEC11 Examiner Reports
WRONG: "On balance, the indirect tax is the most effective policy for correcting the externality." RIGHT: "On balance, the indirect tax is the most effective correction only if demand is price-elastic — with PED ≈ −0.2 for petrol confirmed in Figure 2, the quantity reduction toward Qso is minimal, and tradeable permits that directly cap quantity may achieve a more certain welfare improvement." MARK COST: Level 2 evaluation maximum = max 4/6 eval = max 12/14 total Q12e. One unconditional sentence = entire eval band capped.
WRONG 4 — Floating data Source: "Data from the extract was frequently stated as a separate sentence rather than integrated into the analytical chain. This approach failed to earn Application marks." — Oct 2023 WEC11 Examiner Report
WRONG: "Chicken prices rose 147% between 2020 and 2022. This shows supply fell." RIGHT: "With chicken prices rising 147% — from $1.50 to $3.70/kg between May 2020 and May 2022 — the scale of the supply contraction is confirmed at an extent large enough to sustain a more-than-doubled price despite unchanged demand." TEST: Remove the figure → does argument still make same generic point? YES = floating = zero App.
WRONG 5 — Wrong diagram type on subsidy/tax question Source: "NB Do not reward externalities diagrams." — Jun 2024 WEC11 mark scheme (subsidy question, verbatim)
WRONG: Drawing MSC above MPC externality diagram on a BEB subsidy question. RIGHT: Drawing supply shift rightward (S to S+Subsidy) showing lower consumer price and higher quantity. MARK COST: Zero K marks for diagram. All diagram marks = zero. Cannot recover to Level 3 KAA without correct diagram.
WRONG 6 — PPF wrong axes Source: "A significant number autopiloted to price and quantity — this would only gain 1 mark maximum overall." — Oct 2024 WEC11 Examiner Report (verbatim)
WRONG: Y-axis = "Price", X-axis = "Quantity" on PPF question. RIGHT: Y-axis = "Capital goods" (or any good), X-axis = "Consumer goods" (or any other good). MARK COST: Maximum 1/4 marks regardless of diagram quality.
THE SAME ANSWER AT FOUR LEVELS — EXAMINER ANNOTATED
The question: "With reference to the extract, explain why demand for chicken is price inelastic." (4 marks, Q10 type)
LEVEL 1 (1/4) — What failing students write
[Genuine student register — wrong direction, no formula, confused terminology]
"Demand for chicken is inelastic because people need food. When the price goes up, more people want to buy it. The PED is positive. This shows the market is efficient."
Examiner annotates: Wrong direction (price up → Qd falls, not rises). "People need food" is assertion, not analysis. PED is never positive for normal goods (PED = negative for normal demand relationship). "Market is efficient" is irrelevant. → Level 1: isolated, imprecise knowledge. No chain. 1/4.
LEVEL 2 (2–3/4) — What C-grade students write
[Genuine student register — correct concept, correct direction, stops at Stage 3]
"Price elasticity of demand measures how much quantity demanded changes when price changes. [K ✓] When chicken prices rose from $1.50 to $3.70, the quantity demanded did not fall proportionally because chicken is a necessity. [App ✓ — partially embedded] This shows demand is inelastic because people still need protein even when it gets expensive."
Examiner annotates: K mark present (definition) ✓. Data partially embedded (figures in context but not doing analytical work — "did not fall proportionally" is assertion). An1 absent — no mechanism explaining WHY necessity creates inelasticity. An2 absent — no consequence stated. → Level 2: chains evident but stages omitted. 2–3/4.
ADDS to reach Level 3: The mechanism sentence (An1) and consequence (An2). 25 seconds.
LEVEL 3 ENTRY (3–4/4) — What B-grade students write
[Correct formula, embedded data, mechanism present, consequence stated]
"Price elasticity of demand measures the responsiveness of quantity demanded to a change in price — PED = %ΔQd / %ΔP, and a value between 0 and −1 confirms inelastic demand. [K ✓] With chicken prices rising 147% — from $1.50/kg in May 2020 to $3.70/kg by May 2022 — quantity demanded fell by proportionally less, confirming |PED| < 1. [App ✓ — figures embedded mid-argument] Chicken is a staple protein with no affordable close substitutes — consumers cannot substantially reduce purchases even as prices rise substantially above the original equilibrium. [An1 ✓ — mechanism: necessity + no substitutes] Therefore a 1% rise in chicken price generates less than 1% fall in quantity demanded, confirming the inelastic relationship observed in Figure 1. [An2 ✓]"
Examiner annotates: K mark ✓. App mark ✓ (147%, $1.50, $3.70 doing analytical work — removing them weakens argument). An1 ✓ (mechanism: necessity + no substitutes). An2 ✓ (consequence stated: PED < 1 confirmed). → Level 3. 4/4.
TRANSITION FROM LEVEL 2: Added An1 (necessity mechanism) + An2 (PED confirmation). Two sentences. 25 seconds.
LEVEL 3 TOP (4/4 with maximum depth) — What A*-grade students write
[Everything in Level 3 entry PLUS: diagram letter reference, determinant named, consequence extended]
"Price elasticity of demand measures the responsiveness of quantity demanded to a change in price — PED = %ΔQd / %ΔP, and |PED| < 1 confirms inelastic demand where percentage quantity change is smaller than percentage price change. [K ✓] With chicken prices rising 147% — from $1.50/kg to $3.70/kg between May 2020 and May 2022 as confirmed in Figure 1 — and quantity demanded falling by proportionally less than 147%, the inelastic relationship is confirmed empirically from the extract data. [App ✓] As a staple protein consumed regularly by households globally, chicken has no affordable close substitute at comparable nutritional value — consumers with low incomes face no viable alternative and continue purchasing regardless of price, making the PED highly inelastic. [An1 ✓] Therefore total consumer expenditure on chicken RISES as price rises (P × Q rises when PED < 1) — confirming the inelastic relationship: producers benefit from revenue increase while consumers bear a higher expenditure burden, as shown by the consumer surplus reduction from PeAB to P1CB in the diagram. [An2 ✓ — extended to revenue consequence]"
ADDS vs Level 3 entry: Extended An2 to include total expenditure consequence (P×Q rises when PED<1) and diagram letter reference. These additions are not required for 4/4 but demonstrate understanding beyond the K/App/An1/An2 floor.
EXAMINER THOUGHT PROCESS — THREE STAGES FOR EVERY RULE
For every major rule, the examiner's decision process follows three stages. Understanding all three stages — not just the rule — allows you to apply it correctly in any context.
RULE: Stage 4 earns the An2 mark
STAGE 1 — WHAT THE EXAMINER READS: "The examiner picks up a script and reads: 'Carbon emissions from chicken production harm third parties.' They note: Stage 3 endpoint. Chain stops before welfare consequence."
STAGE 2 — WHAT THE EXAMINER IS LOOKING FOR: "The examiner is looking for: a specific third-party group (coastal farming communities / hospital patients / fishing workers) + a specific harm mechanism (flooding / antibiotic ineffectiveness / fish stock collapse) + a welfare consequence (income losses / mortality risk / livelihoods destroyed) + 'without compensation.' Until all four are present, An2 is not awarded."
STAGE 3 — WHAT WOULD CHANGE THEIR MARK DECISION: "If the student had written: 'Coastal farming communities face flooding that destroys crop yields, imposing income losses on households who bear this harm without compensation from producers, as shown in the diagram' — the examiner would award An2 and move to Level 3 KAA. That sentence took 20 seconds to write."
RULE: Unconditional conclusion = Level 2 evaluation
STAGE 1 — WHAT THE EXAMINER READS: "The examiner reads: 'On balance, the indirect tax is the most effective policy for correcting the externality.' They note: unconditional. No named condition under which this conclusion fails."
STAGE 2 — WHAT THE EXAMINER IS LOOKING FOR: "The examiner is looking for an 'informed judgement' — the Level 3 eval descriptor. An informed judgement is, by definition, conditional. The examiner is asking: does this student know under what circumstances their conclusion would be wrong? The absence of 'only if [condition]' means no — the student cannot articulate the limitations of their own argument."
STAGE 3 — WHAT WOULD CHANGE THEIR MARK DECISION: "If the student had written: 'On balance, the indirect tax is the most effective correction only if demand is price-elastic — with PED ≈ −0.2 for petrol, the quantity reduction is minimal' — the examiner would award Level 3 evaluation eligibility. Three words: 'only if [condition].'"
RULE: App marks require embedded data not cited data
STAGE 1 — WHAT THE EXAMINER READS: "The examiner reads: 'Chicken prices rose 147% between 2020 and 2022. This shows supply fell.' They note: data stated as separate sentence. Not integrated into the mechanism."
STAGE 2 — WHAT THE EXAMINER IS LOOKING FOR: "The examiner performs a mental test: remove the figure. Does the argument still make the same generic point? In this case: 'Chicken prices rose. This shows supply fell.' — same generic argument. The figure was decorative, not analytical. App mark = zero."
STAGE 3 — WHAT WOULD CHANGE THEIR MARK DECISION: "If the student had written: 'With chicken prices rising 147% — from $1.50 to $3.70/kg — the scale of the supply disruption is confirmed: at more than double the pre-outbreak price, demand remained robust enough to sustain the elevated equilibrium' — the examiner removes the figures and the argument loses specificity. App mark awarded."
FOUR LEVELS — PPF DIAGRAM (Most Common Section B Error)
Question: "Draw a production possibility frontier (PPF) diagram to show the effect of an improvement in technology." (4 marks)
LEVEL 1 — Wrong axes, no shift shown: [Diagram with Y-axis = "Price", X-axis = "Quantity", single curve, no shift] → Y-axis = Price: confirmed maximum 1/4 (Oct 2024 examiner: "autopiloted to price and quantity — 1 mark maximum"). X-axis = Quantity: same error. No outward shift. Level 1.
LEVEL 2 — Correct axes, shift in wrong direction: [Diagram with Y-axis = "Capital goods", X-axis = "Consumer goods", curve shifts INWARD] → Axes correct ✓. But: technology improvement → OUTWARD shift (more of both goods possible). Inward shift = economic contraction. K1 ✓ (axes correct). App1 zero (wrong direction). Level 2.
LEVEL 3 ENTRY — Correct axes, correct outward shift: [Diagram with Y-axis = "Capital goods", X-axis = "Consumer goods", original PPF₀ and new PPF₁ shifted OUTWARD. Both axes labelled. Both curves labelled.] → K1 ✓ (correct diagram type, correct axes). App1 ✓ (outward shift). App2 ✓ (original and new PPF labelled). App3 partial (no arrow showing direction of shift). Level 3 entry — 3/4.
LEVEL 3 TOP — Axes, outward shift, equilibrium points, no text: [Same as entry, PLUS: arrow on outward shift. Point A on PPF₀ labelled. Point B on PPF₁ labelled. "PPF₁" label. Zero written text alongside.] → K1 ✓. All App marks ✓. No text (confirmed by Jun 2022: "all marks achievable through diagram; text did not earn additional credit"). 4/4.
EXAMINER 3-STAGE — PPF AXES: STAGE 1 — The examiner looks at the PPF diagram. First check: axis labels. Price and Quantity appear. STAGE 2 — The examiner marks: "NB maximum 1 mark overall for a diagram using price and quantity as axes." All other marks on the diagram become inaccessible regardless of diagram quality. STAGE 3 — If the axes read "Capital goods" and "Consumer goods" (or any two goods) → axis check passes → K marks accessible. This is the single highest-mark-cost error on Draw questions.
FOUR LEVELS — EXTERNALITY DIAGRAM
LEVEL 1: S/D diagram with supply shifting when externality question asked → Wrong diagram type: question asks for externality (MSC/MSB diagram), student draws S/D. Zero K marks. Level 1.
LEVEL 2: MSC above MPC direction correct, but axes wrong ("Price" / "Quantity") → Direction correct ✓. Wrong axes label (should be "Costs and benefits" / "Quantity"). K1 partially awarded. Level 2.
LEVEL 3 ENTRY: Correct diagram, correct axes ("Costs and benefits" / "Quantity"), MSC above MPC, Qme right of Qso, welfare triangle shown but unlabelled → K1 ✓, App1 ✓, App2 ✓. Welfare triangle present but not labelled A/B/C → App3 partial. Level 3 entry.
LEVEL 3 TOP: All above PLUS welfare triangle labelled A/B/C, Qso and Qme both labelled with dotted lines to axes, Pe labelled at social optimum → All marks. 4/4. Level 3 top.
→ Also read: CODEX · KAA Sentence Bank (ready-made chains) | FORGE · Self-Mark Checklist (post-practice diagnosis) | SIGNAL · Predicted Questions 2026 (what to expect)
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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 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 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
The specific missing element — not a category. "Needs more analysis" is useless. "Missing Stage 4: WHO + WHAT + WELFARE in one sentence" is the diagnosis.
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.
PART 6 — WRONG VS RIGHT: ONE FOR EVERY DIAGRAM TYPE
Drills that cannot be self-marked teach nothing. Every drill in this section has a binary PASS/FAIL test applied immediately after completing it.
DIAGRAM TYPE 1 — Negative externality (MSC/MPC)
WRONG: MSC drawn BELOW MPC. Social optimum to the LEFT of market equilibrium. → Zero K marks. External costs ADD to private cost → MSC always ABOVE MPC. Overproduction = Qme to the RIGHT of Qso. Left = underconsumption = positive externality direction.
RIGHT: MSC above MPC throughout. Qme (MPB=MPC intersection) to the RIGHT of Qso (MSB=MSC intersection). Welfare loss triangle between Qso and Qme — units produced that impose MSC > MSB.
Axis check: Y = "Costs and benefits" (exact — not "Price"). X = "Quantity". Both labelled.
DIAGRAM TYPE 2 — Positive externality (MSB/MPB)
WRONG: MSB drawn BELOW MPB. Social optimum to the LEFT of market equilibrium. → Same error mirror-image. External benefits ADD to private benefit → MSB always ABOVE MPB. Underconsumption = Qme to the LEFT of Qso.
RIGHT: MSB above MPB throughout. Qme to the LEFT of Qso. Welfare loss triangle between Qme and Qso — units NOT consumed that would generate MSB > MSC.
DIAGRAM TYPE 3 — Specific indirect tax
WRONG: Supply curve PIVOTS (rotates, changes slope). → Pivot = ad valorem tax (percentage of price). Specific tax = fixed £ per unit regardless of price → PARALLEL shift (same vertical distance at every point on the supply curve).
RIGHT: Supply shifts LEFT in PARALLEL — the vertical distance between S and S+Tax is constant at every quantity. Not a pivot. Not a rightward shift.
Memory rule: Specific → Parallel shift. Ad Valorem → Angled (pivot).
DIAGRAM TYPE 4 — Subsidy
WRONG: Demand curve shifts right for a producer subsidy. → Producer subsidy reduces PRODUCTION COSTS → SUPPLY shifts right. Demand is unchanged — the consumer did not receive the subsidy.
RIGHT: Supply shifts RIGHT from S to S+Sub. Consumer price FALLS (Pe → P1). Quantity RISES (Qe → Q1). Producer effective price RISES above market price (includes subsidy component).
DIAGRAM TYPE 5 — Maximum price
WRONG: Maximum price line drawn ABOVE equilibrium. → A maximum price above Pe has no effect — the market already clears below the ceiling. No shortage. No diagram content worth marking.
RIGHT: Maximum price at Pmax BELOW Pe. At Pmax: Qs contracts (supply below Qe), Qd extends (demand above Qe). Shortage = Qd − Qs. Arrow showing shortage gap.
DIAGRAM TYPE 6 — Minimum price
WRONG: Minimum price line drawn BELOW equilibrium. → A minimum price below Pe has no effect — the market already clears above the floor.
RIGHT: Minimum price at Pmin ABOVE Pe. At Pmin: Qs extends (supply above Qe), Qd contracts (demand below Qe). Surplus = Qs − Qd. Arrow showing surplus gap.
DIAGRAM TYPE 7 — PPF
WRONG: Y-axis = "Price" or "Price level". X-axis = "Quantity" or "Real output". → Maximum 1 mark. "A significant number autopiloted to price and quantity and this response would only gain 1 mark maximum overall." — Oct 2024 WEC11 Examiner Report.
RIGHT: Y-axis = "Capital goods" (or "Good A" or any output type). X-axis = "Consumer goods" (or "Good B" or any different output type). NEVER macroeconomic axes on a microeconomic PPF.
DIAGRAM TYPE 8 — Two-curve price change (Q12c type)
WRONG: Demand shift on one diagram, supply shift on second separate diagram. → Final equilibrium mark automatically lost. Two separate diagrams cannot show the combined new equilibrium.
RIGHT: Both curves shift on THE SAME diagram. D shifts to D1. S shifts to S1. Final equilibrium at intersection of D1 and S1 with dotted lines to BOTH axes (P1 and Q1). Both old equilibrium (Pe, Qe) and new (P1, Q1) labelled.
PART 7 — DIAGNOSE YOUR DIAGRAM
ATTEMPT 1 — Tax question: Student draws demand curve shifting LEFT for an indirect tax. → Wrong curve. Indirect tax raises production costs → SUPPLY shifts left. Demand is not affected by the tax directly (the consumer still has the same preferences at every price).
Upgrade: Always identify WHICH curve is affected. "Tax on producers → S shifts." "Tax on consumers → D shifts." "Producer subsidy → S shifts right." Demand shifts only. When consumer income, prices of related goods, or tastes change.
ATTEMPT 2 — Externality question: Student draws a standard supply and demand diagram for a negative externality question, showing S shifting left (as if a tax was imposed) without MSC/MPC curves. → Wrong diagram type. The negative externality diagram requires MSC above MPC — NOT a supply shift. The supply curve IS the MPC. The MSC must be drawn. As a SEPARATE curve above the supply/MPC curve.
Upgrade: Draw S = MPC upward sloping. Draw MSC above it (also upward sloping, further from origin). Draw D = MPB = MSB downward sloping (if no external benefits). Market equilibrium. Where D crosses S. Social optimum where D crosses MSC. Welfare loss triangle between them.
ATTEMPT 3 — PPF question: Student draws outward shift of PPF with Y-axis "Price level" and X-axis "Real output." → These are AD/AS axes not PPF axes. 1 mark maximum.
Upgrade: Cross out and redraw with Y = "Capital goods", X = "Consumer goods." The axes are everything on a PPF question.
ATTEMPT 4 (full marks on any diagram type): Any diagram with: (1) correct diagram type for the question, (2) correct axis labels specific to the good/context, (3) original equilibrium with dotted lines to both axes, (4) correct shift direction with the curve labelled (D1, S1, S+Tax, MSC, Pmax), (5) new equilibrium with dotted lines to both axes, (6) any specific element asked for (welfare triangle, shortage, surplus) labelled. Zero written text alongside (on Draw questions).
REFERENCE CARD — DIAGRAM QUICK-DRAW
AXES RULES (memorise):
Externality: Y="Costs and benefits" X="Quantity"
Tax/Subsidy: Y="Price of [good]" X="Quantity"
PPF: Y="[Good A]" X="[Good B]"
NEVER "Price" or "Quantity" on PPF
CURVE DIRECTIONS:
Neg ext: MSC ABOVE MPC Pos ext: MSB ABOVE MPB
Tax: S shifts LEFT Subsidy: S shifts RIGHT
Max price: line BELOW Pe Min price: line ABOVE Pe
Tax shift: PARALLEL (specific) | PIVOT (ad valorem)
WRONG type diagram = ZERO K marks
BOTH SHIFTS → ONE DIAGRAM (Q12c rule)
Separate = K2 (final equilibrium) lost
EMERGENCY (5 min left): Write "only if [condition]" NOW.
Q12e: judgement first, then complete chains. Section D: same.
Never leave eval blank — 2-4 marks saved in 30 seconds.
If you found this useful → also read: FORGE · 4-Mark System (Draw questions in Section B) | FORGE · 8-Mark System (diagrams in 8-mark context) | FORGE · Chain Engine (how to reference diagram in writing)
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VERIDIAN WEC11 · FORGE SERIES · THE DIAGRAM MASTERCLASS
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