Use the AD/AS model to determine equilibrium price level and output
Analyse the effects of demand-side and supply-side shocks on equilibrium, distinguishing short-run from long-run outcomes
Explain inflationary and deflationary gaps and how the economy may (or may not) self-correct
Evaluate the Keynesian vs Classical perspectives on adjustment and the role of government policy
The AD/AS Model
The AD/AS Model — Determining Equilibrium
AD — downward sloping
SRAS — upward sloping
LRAS — vertical at Yf
Equilibrium E (P*, Y*)
EQUILIBRIUM
Occurs where AD = SRAS. At E, the price level (P*) and output level (Y*) are simultaneously determined. If AD > SRAS: upward pressure on prices and output expands. If AD < SRAS: downward pressure on prices, firms cut output.
FULL EMPLOYMENT
LRAS shows potential output (Yf). If Y* = Yf: economy at full employment. If Y* < Yf: negative output gap (recessionary). If Y* > Yf: positive output gap (inflationary).
THREE POSSIBLE POSITIONS
(1) Below potential (Y* < Yf): negative output gap — Keynesian concern. (2) At potential (Y* = Yf): full employment equilibrium — macro ideal. (3) Above potential (Y* > Yf): temporary boom — inflationary pressure, will self-correct (Classical) or require policy tightening.
Demand-Side Shocks
Positive Demand-Side Shock
AD1 (original)
AD2 (shifted right)
SRAS shifts left (LR)
Inflationary gap (Y2 > Yf)
SHORT-RUN EFFECT
AD rises (e.g. government stimulus, consumer confidence boom) → AD2 gives higher P and higher Y. If economy has spare capacity (Y < Yf), most of the effect is on output. If near full employment, most effect is on price level — inflationary.
THE INFLATIONARY GAP
If Y* > Yf, firms face labour and capital shortages. Wages bid up → SRAS shifts left → in the long run, output returns to Yf at even higher price level E3. Classical conclusion: demand stimulus is ultimately only inflationary if economy is at or above potential.
KEYNESIAN CASE
If economy has large spare capacity (far below Yf), the AS curve is relatively flat → demand increase raises output substantially with little price pressure. Multiplier effect magnifies the initial injection. This is the Keynesian argument for stimulus in deep recessions.
Demand-Side Shocks
Negative Demand-Side Shock
AD1 (original)
AD2 (shifted left)
SRAS shifts right (Classical LR)
Deflationary gap (Y2 < Yf)
SHORT-RUN
AD falls (recession, credit crunch, confidence collapse). Price level falls, output falls, unemployment rises. Deflationary gap emerges: Y* < Yf — the economy is operating below potential.
CLASSICAL SELF-CORRECTION
Falling demand → wages fall (flexible) → SRAS shifts right → economy returns to Yf at lower price level (E3). No need for government intervention — the market clears automatically.
KEYNESIAN CRITIQUE
Wages are sticky downward — workers and unions resist wage cuts. Self-correction is slow, painful, or non-existent. 1930s Great Depression: economy stuck well below potential for over a decade. Keynes: "In the long run we are all dead." Government must intervene to restore AD.
Supply-Side Shocks
Negative Supply-Side Shock — Stagflation
SRAS1 (original)
SRAS2 (cost shock shifts left)
AD (unchanged)
STAGFLATION
Simultaneous inflation and falling output. Negative supply shocks → firms face higher costs → pass on to prices (inflation) and cut output (recession). 1973 OPEC oil crisis: SRAS shifted sharply left → UK inflation hit 25%, GDP fell. 2022 energy/food price shock: same mechanism.
THE POLICY DILEMMA
Boost AD (cure recession): output restored but price level rises further — makes inflation worse. Tighten policy (cure inflation): price level falls but output falls further — makes recession worse. No AD policy can simultaneously fix both. Supply-side policies needed to shift SRAS back right.
MONETARY POLICY RESPONSE 2022
Bank of England raised rates (tightening → AD left) to fight inflation despite recession risk. A "sacrifice" — accepting lower output to achieve price stability. BofE's 2% CPI mandate forced it to prioritise inflation over growth, illustrating the stagflation dilemma in real time.
Genuine improvement in productive potential (new technology, capital accumulation, better human capital) shifts LRAS right. This is non-inflationary growth — more output at same or lower price level. The core objective of supply-side policy.
EXAMPLES
1990s internet revolution shifted LRAS right in the US → sustained growth with low inflation (the "Goldilocks economy"). North Sea oil (1970s–80s) shifted UK LRAS right. Potential of AI: expected to raise TFP → significant LRAS shift over coming decades.
EVALUATION
Long time lags — investment in physical and human capital takes years to feed through. Policy uncertainty undermines private investment decisions. The UK "productivity puzzle" shows that even supply-side reforms don't automatically translate into productivity gains.
Real-World Applications
Case Studies: AD/AS in Action
UK 2008–09 FINANCIAL CRISIS
AD collapsed (credit crunch, banking crisis, confidence collapse). AD shifted sharply left → deepest recession since WWII (GDP −6%). Government response: fiscal stimulus (car scrappage, VAT cut), Bank Rate cut to 0.5%, QE. Partially offset AD fall but long-lasting negative output gap remained.
COVID-19 (2020)
Simultaneous AD shock (lockdowns → consumer spending fell) and supply shock (supply chains disrupted, labour supply reduced). AD and SRAS both shifted left → complex, asymmetric recovery. Furlough scheme: government maintained incomes → supported C → limited AD fall.
THE 2021–23 INFLATION SURGE
Post-COVID pent-up demand (AD shifted sharply right) met supply-constrained economy (SRAS still left from disruptions). Classic AD > SRAS overheating. Then Ukraine war (further negative SRAS shock). Result: 11.1% CPI. AD/AS perfectly describes the mechanism: demand surge + supply restriction = inflation.
CONFLICTING OBJECTIVES
Expansionary policy curing a deflationary gap may create an inflationary gap if it overshoots. UK 2020–22: government stimulus (necessary) → AD overshot SRAS capacity → inflation. Timing and calibration of policy are crucial and extremely difficult — genuine macro uncertainty.
Perspectives & Policy
Self-Correction vs Policy Intervention
CLASSICAL (NEO-CLASSICAL) VIEW
Markets clear automatically. Flexible wages and prices ensure the economy self-corrects to LRAS. Recessions are short-lived. Implication: do nothing — let the market mechanism restore equilibrium. Government intervention distorts signals and slows adjustment.
KEYNESIAN VIEW
Markets can fail to self-correct. Sticky wages (especially downward), animal spirits (confidence traps), liquidity traps → economy can be stuck in a deflationary equilibrium indefinitely. Government must actively manage AD to restore full employment.
MONETARIST MIDDLE GROUND (FRIEDMAN)
Agreed with Classical long-run vertical LRAS. But: self-correction takes time, and government AD management based on imprecise forecasts → policy errors add instability. Preferred: stable, rule-based policy (money supply growth rule; later adopted as inflation targeting).
MODERN CONSENSUS
Most central banks operate "flexible average inflation targeting" — broadly Keynesian in recessions (stimulus), broadly Classical near full employment (restraint). The AD/AS model is the central framework used by the BofE, OBR, and Treasury. Debate is now about magnitudes, not fundamentals.
Evaluation
Evaluating the AD/AS Model
For (AD/AS model usefulness)
Provides a unified framework for price level and output analysis simultaneously
Distinguishes short-run from long-run effects clearly — essential for policy analysis
Explains both demand AND supply shocks and their contrasting effects
Helps justify policy responses: stimulus in deflationary gap; tightening in inflationary gap
Against (limitations)
Simplified model — assumes distinct AD and AS when in reality they interact
Keynesian vs Classical shape of LRAS remains genuinely contested
Output gaps are unobservable and estimates differ widely between forecasters
Policy lags mean interventions are often mistimed, potentially worsening instability
Essay Tip: "The best AD/AS essays specify WHERE on the AS curve the economy sits. Near the horizontal (Keynesian) portion: stimulus raises output, not prices. Near the vertical (Classical/full employment): stimulus is inflationary. Always state the economic context before applying the model."
Glossary
Key Terms
Macroeconomic Equilibrium
The price level and output at which AD = SRAS. At this point there is no tendency for the price level or output to change. May or may not coincide with full employment (Yf).
Inflationary Gap
When actual output (Y*) exceeds potential output (Yf). The economy is producing beyond sustainable capacity, creating upward pressure on wages and prices. Classical economists predict SRAS will self-correct leftward.
Deflationary Gap
When actual output (Y*) is below potential output (Yf). Unemployment above the natural rate. Keynesians argue government stimulus is required; Classicists argue wages will fall and SRAS will shift right automatically.
Stagflation
The simultaneous occurrence of stagnation (falling output, rising unemployment) and inflation. Caused by a negative supply shock shifting SRAS left. Creates an insoluble dilemma for demand-side policy.
Self-Correction Mechanism
The Classical view that flexible wages and prices automatically restore equilibrium at Yf following a shock. Keynesians dispute this due to wage stickiness and the possibility of long-run deflationary traps.
Output Gap
The difference between actual GDP and potential GDP (Yf). A negative output gap indicates spare capacity and unemployment. A positive output gap indicates inflationary pressure. Output gaps are unobservable and must be estimated.
Question 1 of 8 · Macroeconomic Equilibrium
In the AD/AS model, macroeconomic equilibrium occurs where:
A
AD equals LRAS — the economy is always at full employment
B
AD equals SRAS — the price level and output are simultaneously determined
C
SRAS equals LRAS — the economy is on the production possibility frontier
D
The government balances its fiscal budget — G equals T
Answer · Question 1
In the AD/AS model, macroeconomic equilibrium occurs where:
A
AD equals LRAS — the economy is always at full employment
B
AD equals SRAS — the price level and output are simultaneously determined
C
SRAS equals LRAS — the economy is on the production possibility frontier
D
The government balances its fiscal budget — G equals T
Correct: B. Macroeconomic equilibrium is where AD = SRAS. At this intersection, there is no tendency for the price level or output to change. Crucially, this equilibrium may be below, at, or above potential output (Yf) — it does not require the economy to be at full employment. Option A confuses LRAS with SRAS; the LRAS simply marks potential output, not the equilibrium condition.
Question 2 of 8 · Macroeconomic Equilibrium
A positive demand-side shock occurs when the economy is already at full employment (Y* = Yf). According to the Classical model, what is the long-run outcome?
A
Output rises permanently above Yf — the economy reaches a new higher potential
B
SRAS shifts left as wages are bid up, returning output to Yf at a higher price level
C
The government must cut taxes to sustain the higher output level
D
LRAS shifts right as firms invest in new capacity
Answer · Question 2
A positive demand-side shock occurs when the economy is already at full employment (Y* = Yf). According to the Classical model, what is the long-run outcome?
A
Output rises permanently above Yf — the economy reaches a new higher potential
B
SRAS shifts left as wages are bid up, returning output to Yf at a higher price level
C
The government must cut taxes to sustain the higher output level
D
LRAS shifts right as firms invest in new capacity
Correct: B. When the economy is at full employment (Y* = Yf), a positive demand shock creates an inflationary gap (Y > Yf). Labour and capital are scarce — workers bid up wages. Rising costs shift SRAS left. In the long run, the economy returns to Yf but at a higher price level. The Classical conclusion: demand stimulus beyond full employment is purely inflationary — it cannot permanently raise real output above potential.
Question 3 of 8 · Macroeconomic Equilibrium
Stagflation is best described as the result of:
A
AD shifting right while SRAS shifts left — demand and supply both deteriorate
B
AD shifting left — causing both falling output and falling prices
C
SRAS shifting left — raising price level and reducing output simultaneously
D
LRAS shifting left — reducing the economy's long-run productive potential
Answer · Question 3
Stagflation is best described as the result of:
A
AD shifting right while SRAS shifts left — demand and supply both deteriorate
B
AD shifting left — causing both falling output and falling prices
C
SRAS shifting left — raising price level and reducing output simultaneously
D
LRAS shifting left — reducing the economy's long-run productive potential
Correct: C. Stagflation (stagnation + inflation) is uniquely caused by a negative supply shock shifting SRAS left. Because SRAS slopes upward, a leftward shift means the AD curve intersects it at a higher price level AND lower output — simultaneously producing inflation and recession. A demand-side shock (option B) cannot do this: a leftward AD shift reduces both output and price level. Option A is incorrect — AD shifting right would boost output, partially offsetting the SRAS effect.
Question 4 of 8 · Macroeconomic Equilibrium
Classical economists argue that during a recession (Y* < Yf) the economy will:
A
Remain permanently below Yf — recessions are irreversible without government action
B
Require fiscal stimulus to shift AD rightward back to Yf
C
Self-correct as falling wages shift SRAS right, restoring Yf at a lower price level
D
Self-correct through rising consumer confidence boosting AD automatically
Answer · Question 4
Classical economists argue that during a recession (Y* < Yf) the economy will:
A
Remain permanently below Yf — recessions are irreversible without government action
B
Require fiscal stimulus to shift AD rightward back to Yf
C
Self-correct as falling wages shift SRAS right, restoring Yf at a lower price level
D
Self-correct through rising consumer confidence boosting AD automatically
Correct: C. The Classical self-correction mechanism: in a recession, excess labour supply → wages fall → firms' costs fall → SRAS shifts right → output returns to Yf at a lower price level. This is the supply-side adjustment mechanism. Option D describes an AD mechanism, which is Keynesian animal spirits logic, not Classical price/wage flexibility. Classicists explicitly argue government stimulus (option B) is unnecessary and potentially harmful.
Question 5 of 8 · Macroeconomic Equilibrium
The Keynesian view argues the economy can remain stuck below Yf because:
A
Prices are perfectly flexible but wages are fixed by government legislation
B
Wages are sticky downward — workers and unions resist wage cuts, preventing SRAS from shifting right
C
The LRAS curve is horizontal in the Keynesian model — so output is always determined by supply alone
D
The Bank of England always raises interest rates in recessions, preventing AD from recovering
Answer · Question 5
The Keynesian view argues the economy can remain stuck below Yf because:
A
Prices are perfectly flexible but wages are fixed by government legislation
B
Wages are sticky downward — workers and unions resist wage cuts, preventing SRAS from shifting right
C
The LRAS curve is horizontal in the Keynesian model — so output is always determined by supply alone
D
The Bank of England always raises interest rates in recessions, preventing AD from recovering
Correct: B. The key Keynesian insight is downward wage stickiness. Workers resist nominal wage cuts (money illusion, union bargaining, efficiency wage considerations). Therefore, the Classical self-correction mechanism fails: if wages don't fall, SRAS cannot shift right, and the economy remains stuck below Yf. This is why Keynes advocated government intervention — fiscal stimulus to shift AD right rather than waiting for wages to fall. Option C is wrong: Keynesians argue the AS curve is horizontal at low output levels (not LRAS), meaning AD determines output there.
Question 6 of 8 · Macroeconomic Equilibrium
A deflationary gap exists when:
A
The price level is falling — deflation is occurring in the economy
B
Actual output (Y*) exceeds potential output (Yf) — the economy is overheating
C
Actual output (Y*) is below potential output (Yf) — the economy has spare capacity
D
The government runs a budget deficit — spending exceeds tax revenues
Answer · Question 6
A deflationary gap exists when:
A
The price level is falling — deflation is occurring in the economy
B
Actual output (Y*) exceeds potential output (Yf) — the economy is overheating
C
Actual output (Y*) is below potential output (Yf) — the economy has spare capacity
D
The government runs a budget deficit — spending exceeds tax revenues
Correct: C. A deflationary gap (also called a recessionary or negative output gap) occurs when Y* < Yf — actual output is below potential, unemployment is above the natural rate, and there is spare capacity in the economy. Note: a deflationary gap does not necessarily mean deflation (falling price level) is occurring — prices can still be rising, just more slowly. Option B describes an inflationary gap (Y* > Yf), not a deflationary one.
Question 7 of 8 · Macroeconomic Equilibrium
The UK inflation surge of 2021–23 (peak 11.1% CPI) is best analysed in the AD/AS model as:
A
A purely demand-side shock — AD shifted right due to excess money supply growth
B
A purely supply-side shock — SRAS shifted left due to energy and food price rises
C
A combined shock — AD shifted right (post-COVID demand) while SRAS shifted left (supply disruptions and energy costs), driving exceptional inflation
D
A wage-price spiral — workers demanded higher wages, shifting LRAS left permanently
Answer · Question 7
The UK inflation surge of 2021–23 (peak 11.1% CPI) is best analysed in the AD/AS model as:
A
A purely demand-side shock — AD shifted right due to excess money supply growth
B
A purely supply-side shock — SRAS shifted left due to energy and food price rises
C
A combined shock — AD shifted right (post-COVID demand) while SRAS shifted left (supply disruptions and energy costs), driving exceptional inflation
D
A wage-price spiral — workers demanded higher wages, shifting LRAS left permanently
Correct: C. The 2021–23 UK inflation had both demand and supply components. AD shifted right: post-lockdown pent-up demand, fiscal stimulus (furlough preserved incomes), and global monetary loosening. SRAS shifted left: global supply chain disruptions (COVID aftermath), energy price shocks (Ukraine war), and Brexit-related trade frictions. Both shifts simultaneously pressed on price level — explaining why UK inflation was so persistent and severe. This is the most accurate AD/AS account and demonstrates why policymakers faced a difficult dilemma.
Question 8 of 8 · Macroeconomic Equilibrium
A rightward shift in LRAS (e.g. due to improved technology) differs from a rightward shift in AD (e.g. due to government stimulus) because:
A
An AD shift permanently raises potential output; an LRAS shift only temporarily raises actual output
B
An LRAS shift raises potential output non-inflationary; an AD shift (near full employment) raises output and the price level, risking inflation
C
An LRAS shift only benefits firms; an AD shift benefits all households equally
D
Both shifts have identical effects on output and price level in the long run
Answer · Question 8
A rightward shift in LRAS (e.g. due to improved technology) differs from a rightward shift in AD (e.g. due to government stimulus) because:
A
An AD shift permanently raises potential output; an LRAS shift only temporarily raises actual output
B
An LRAS shift raises potential output non-inflationary; an AD shift (near full employment) raises output and the price level, risking inflation
C
An LRAS shift only benefits firms; an AD shift benefits all households equally
D
Both shifts have identical effects on output and price level in the long run
Correct: B. A rightward LRAS shift represents genuine growth in productive capacity — the economy can produce more at every price level. This allows output to rise without inflationary pressure (or even with falling prices). By contrast, an AD shift near full employment pushes output above Yf creating an inflationary gap — higher prices result. This distinction is fundamental: supply-side economists argue LRAS shifts are superior to demand stimulus because they raise output without inflation. However, LRAS shifts take much longer to achieve (investment lags, skill development) — the key evaluation point.
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Lesson Complete
You've covered the AD/AS model, macroeconomic equilibrium, positive and negative demand shocks, supply shocks, stagflation, real-world applications (2008, COVID, 2021–23 inflation), Classical vs Keynesian perspectives, and the evaluation of the model.