AD/AS Model & Macroeconomic Equilibrium

The master diagram for all macroeconomic analysis — AD, SRAS, LRAS, the four shift scenarios, output gaps, and the Keynesian vs Classical debate

SPEC 2.3 • THE UK ECONOMY ⭐ HIGH PRIORITY

Why This Is the Most Important Diagram

The AD/AS model is the single most versatile diagram in the entire A Level Economics course. Every macroeconomic essay on Paper 2 and Paper 3 requires it. Every policy question — fiscal, monetary, supply-side — is analysed through it.

What You Must Be Able to Do

Draw the full model (AD, SRAS, LRAS) from memory with all labels

Show all four shift scenarios and their equilibrium effects

Identify and explain output gaps (positive and negative)

Distinguish Keynesian vs Classical LRAS and explain the implications

Apply the diagram to any policy or shock in the question context

Examiner Alert — Caps Your Mark

For any question requiring AD/AS analysis, a valid diagram is mandatory for Level 3+ KAA. Must show: correct shift direction, both old and new equilibria labelled (P₁, Y₁ → P₂, Y₂), Yf marked, and diagram referenced explicitly in written analysis. Missing any of these caps your response at Level 2.

Aggregate Demand — Definition & Components

Aggregate Demand (AD)

The total level of planned expenditure on goods and services produced in an economy at a given price level over a given time period.

AD = C + I + G + (X − M)

Components

C — Consumer spending (≈60% of UK AD)

I — Investment by firms (capital goods)

G — Government spending on goods & services

X − M — Net exports (exports minus imports)

Why AD Slopes Downward

Real wealth effect: higher price level erodes real value of savings → C falls

Interest rate effect: higher prices → more money needed → interest rates rise → I falls

International competitiveness: higher prices → exports less competitive → X falls, M rises

AD Shifters — Any of C, I, G, (X−M) changing

Consumer confidence ↑, interest rates ↓, government spending ↑, exchange rate depreciation, trading partner growth — all shift AD right. The reverse of each shifts AD left.

Aggregate Supply — SRAS and LRAS

SRAS — Short-Run AS

Total output firms are willing to supply at each price level, with wages and other input costs fixed. Slopes upward — higher prices make production more profitable → more supplied.

LRAS — Long-Run AS

Total output when all factors of production are fully and efficiently employed — the economy's productive capacity. Drawn as a vertical line at Yf (potential/full employment output). Price level does not affect long-run productive capacity.

SRAS Shifters (left = costs ↑)LRAS Shifters (right = capacity ↑)

Wage costs riseImproved technology

Oil / commodity price riseMore/better capital investment

Exchange rate depreciation (import costs)Growth in working population

Indirect taxes rise (e.g. VAT)Better education & skills (human capital)

Productivity fallsInstitutional improvements

Government subsidies removedDiscovery of natural resources

The Full AD/AS Diagram — Draw This Perfectly

This is the diagram you must be able to reproduce from memory in the exam. Every label is required.

Checklist — Every Label Required

Axes: "Price Level" (vertical), "Real Output / GDP" (horizontal) | Curves: AD (downward), SRAS (upward), LRAS (vertical, dashed or solid) | Labels: P₁, Y₁ (equilibrium), Yf (potential output) | Output gap: state whether Y₁ < Yf or Y₁ > Yf

Keynesian vs Classical LRAS

This debate appeared on the October 2020 AS Paper 2 (Q2). It underpins every macro policy argument.

Classical Argument

LRAS is vertical at Yf. In the long run, wages and prices adjust to clear markets → economy always returns to full employment. Demand-side intervention is unnecessary and only inflationary.

Keynesian Argument

LRAS has a horizontal section at low output — wages and prices are sticky downward in recessions. Economy can get stuck below Yf. Government demand-side intervention is both possible and necessary.

AO4 Evaluation: consider…

Classical view supports supply-side policy; Keynesian view supports fiscal/monetary stimulus. The 2008 crisis shifted the debate: governments chose Keynesian-style stimulus. COVID-19 reinforced the case for active government intervention in severe downturns.

Scenario 1: AD Shifts Right — Economy Below Yf

Case 1: Demand-Side Stimulus in Recession

Cause: Fiscal stimulus (↑G), cut in interest rates (↑C and ↑I), rise in consumer confidence, exchange rate depreciation (↑X−M).

Chain of Reasoning — Full Marks

AD shifts right AD₁ → AD₂ → new equilibrium at higher real output Y₂ (GDP growth ✓, unemployment falls ✓) and higher price level P₂ (demand-pull inflation risk ✗). Y₂ is still below Yf → spare capacity remains → multiplied effect as initial injection circulates. This is the standard Keynesian demand management case.

Scenario 2: SRAS Shifts Left — Supply-Side Shock

Case 2: Stagflation — The Policy Dilemma

Cause: Oil price spike, rising wage costs, supply chain disruption, exchange rate depreciation raising import costs.

The Stagflation Policy Dilemma

SRAS shifts left → lower output Y₂ AND higher price level P₂ simultaneously = stagflation. Policy dilemma: demand-side expansion (↑G or ↓interest rates) would raise output but worsen inflation. Demand contraction would reduce inflation but worsen unemployment. Only supply-side policy can resolve both simultaneously — by shifting SRAS back right.

Scenarios 3 & 4: Supply-Side Growth & Overheating

Supply-Side Success

LRAS + SRAS both shift right → more output AND lower price level. The supply-side ideal: growth without inflation. Takes years — education reform, infrastructure, R&D.

Overheating Economy

AD₂ beyond LRAS → short-run: output above Yf, inflation rising sharply. Long-run (Classical): wages bid up → SRAS shifts left → economy returns to Yf at permanently higher price level. Purely inflationary — contractionary policy needed.

Output Gaps — The Two Types

Negative Output Gap

Actual GDP < Potential GDP. Economy has spare capacity. Unemployment above natural rate. Deflationary pressure. Firms operating below full capacity.

Policy response: Expansionary demand-side policy (↑G, ↓interest rates) — Keynesian case for stimulus.

Positive Output Gap

Actual GDP > Potential GDP. Economy is overheating. Inflationary pressure. Unemployment below natural rate. Bottlenecks appearing.

Policy response: Contractionary policy (↑interest rates, ↓G) — reduce demand-pull inflationary pressure.

Output Gap Formula

Output gap (%) = [(Actual GDP − Potential GDP) / Potential GDP] × 100

A negative result = negative gap. A positive result = positive gap. The OBR and Bank of England publish official output gap estimates for the UK.

Chain of Reasoning — Negative Gap → Policy

Economy at Y₁ < Yf → negative output gap → cyclical unemployment exists → deflationary gap → government could increase G to shift AD right → multiplier effect → GDP rises toward Yf → unemployment falls → but risk of demand-pull inflation if Y approaches Yf.

Macroeconomic Policy Conflicts — via AD/AS

The AD/AS model reveals why pursuing one objective often conflicts with another. This is essential AO4 for Paper 2 essays.

ConflictAD/AS MechanismTrade-off

Growth ↑ vs Inflation ↑AD shifts right → Y rises toward Yf but P also risesCannot have more growth without some inflationary pressure as Y approaches Yf

Growth ↑ vs BOP deficit ↑Higher Y → more imports → X−M worsens → AD falls back slightlyExpansionary policy that boosts growth also worsens the current account

Low unemployment vs Low inflationTight labour market → wage bargaining → wages rise → SRAS shifts left → P risesPhillips curve trade-off: lower unemployment raises inflationary pressure

Low inflation vs GrowthHigher interest rates to reduce AD → P falls → but Y also falls → growth lostThe cost of price stability is lower short-run growth

Fiscal balance vs GrowthAusterity (↓G) → AD shifts left → Y falls → negative multiplier effectReducing deficit requires cutting spending that supports growth

AO4 Evaluation: consider…

Not all conflicts are inevitable. Supply-side policy can shift LRAS right — raising Y AND reducing P simultaneously. This resolves the growth-inflation conflict without the trade-off of demand-side policy.

Practice Question 1 — 2 Marks (Oct 2020 Style)

In August 2018 the Bank of England raised the base interest rate from 0.5% to 0.75%.

Annotate the AD/AS diagram to show the possible impact of this rise in the base interest rate on the equilibrium level of real output and the price level. (2 marks)

1

Shift: Higher interest rate → borrowing more expensive → C falls (mortgage costs, consumer credit) → I falls (firms' cost of capital rises) → AD shifts left from AD to AD₂. Draw AD₂ to the left of AD, with a leftward arrow labelled.

2

New equilibrium: Mark new intersection of AD₂ and SRAS. Label: new price level P₂ (lower than P₁) and new real output Y₂ (lower than Y₁). Both dashed lines to axes required. Note: 1 mark for correct direction of AD shift; 1 mark for correctly labelled new equilibrium.

Practice Question 2 — 25 Marks

"A rise in aggregate demand is always beneficial for the UK economy."

Assess this view. (25 marks)

Define

AD = C+I+G+(X−M). AD shifts right when any component rises. State that the impact depends critically on the position of the economy relative to Yf and the size of the shift.

KAA 1 — FOR the statement:

If economy has a negative output gap (Y₁ < Yf) → AD shifts right → real output rises toward Yf → GDP growth ✓ → cyclical unemployment falls ✓ → multiplier effect amplifies initial injection → if Y approaches Yf, full employment restored. Diagram: AD₁→AD₂, Y₁→Y₂ (still below Yf), P₁→P₂ (modest rise). Apply: UK COVID recovery 2021 — government stimulus (furlough, spending) shifted AD right, real GDP grew 7.4% in 2021.

KAA 2 — AGAINST the statement:

If economy already at or above Yf → AD shifts right beyond LRAS → short run: output appears above Yf but price level rises sharply (demand-pull inflation) → long run (Classical): wages bid up → SRAS shifts left → economy returns to Yf at permanently higher price level → net effect: no lasting real output gain, only higher price level. Not beneficial. Diagram: AD₂ beyond LRAS, positive output gap, P₂ significantly higher.

EVAL 1 — Depends on position relative to Yf:

A rise in AD during a recession (large negative output gap) is most beneficial — there is spare capacity to absorb the demand without significant inflation. A rise in AD when the economy is near Yf is largely inflationary. The same policy has very different effects depending on the economic cycle.

EVAL 2 — Composition of AD matters:

If AD rises due to ↑I (investment) → productive capacity also expands → LRAS shifts right → more sustainable growth, less inflationary. If AD rises due to ↑C funded by debt → no productive capacity added → purely inflationary risk → unsustainable in long run. Source of AD shift matters for quality of growth.

EVAL 3 — BOP and inflation trade-off:

Higher AD → higher real income → more imports demanded → X−M worsens → current account deficit may widen. Higher domestic prices → exports less competitive → further BOP deterioration. Not "always beneficial" — macro objective conflicts arise.

Judgement

A rise in AD is beneficial when the economy has spare capacity (negative output gap) and when the source of the rise is investment rather than debt-funded consumption. It is harmful when the economy is at or above Yf, when it fuels unsustainable inflation, or when it exacerbates current account imbalances. The statement is too absolute — the benefits of higher AD are highly conditional on economic context.

Summary

Four Scenarios — Master These

ScenarioShiftOutputPrice LevelPolicy Implication

1. Demand stimulus (below Yf)AD →Y rises toward YfP rises moderatelyKeynesian stimulus justified

2. Supply-side shockSRAS ←Y falls (recession)P rises (stagflation)Only supply-side policy resolves

3. Supply-side successLRAS + SRAS →Y risesP fallsGrowth without inflation — ideal

4. OverheatingAD → beyond YfNo LR gainP rises permanentlyContractionary policy needed

Diagram Checklist

Axes: "Price Level" and "Real Output / GDP"

AD (↘), SRAS (↗), LRAS (vertical at Yf)

Label P₁, Y₁ (original) and P₂, Y₂ (new)

State position of Y relative to Yf (output gap)

Reference diagram explicitly in written analysis

Top Evaluation Points

Position relative to Yf determines impact of AD shift

Keynesian vs Classical LRAS shape changes policy conclusions

Stagflation: only supply-side resolves both problems

Supply-side: higher growth AND lower inflation simultaneously

All macro policy involves trade-offs between objectives

Exam Tip — Never Miss These

In any AD/AS question: (1) state the direction of shift and why, (2) show both equilibria on the diagram, (3) state whether Y is above or below Yf, (4) link to at least two macro objectives (e.g. growth AND inflation), (5) evaluate with Keynesian vs Classical or policy conflict. This structure reliably reaches Level 4.