The AD/AS model — Edexcel A-Level Economics B
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The AD/AS model explained
The AD/AS model covers 6 specification points, including a) Changes in Aggregate Demand (AD) and b) Changes in Aggregate Supply (AS).
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Open a point below for its explanation and revision guidance.
a) Changes in Aggregate Demand (AD)
Aggregate Demand (AD) represents the total planned expenditure on goods and services produced within an economy at a given price level over a specific period. It is calculated using the formula AD = C + I + G + (X - M). Changes in AD occur when any of these components alter independently of the general price level, causing the entire AD curve to shift. For example, a reduction in interest rates lowers borrowing costs, stimulating consumer spending (C) and business investment (I), which shifts the AD curve to the right. Conversely, a recession in a major trading partner reduces export demand (X), shifting AD to the left. Understanding these shifts is essential for analysing macroeconomic impacts on real GDP and inflation.
b) Changes in Aggregate Supply (AS)
Aggregate Supply (AS) is the total volume of goods and services producers are willing and able to supply at a given price level. Changes in AS occur in the short run (SRAS) or long run (LRAS). SRAS shifts are caused by changes in production costs, such as wages, raw material prices, or business taxes. For example, a sudden rise in global oil prices increases costs, shifting SRAS to the left. LRAS shifts reflect changes in the economy's productive capacity, driven by the quantity or quality of factors of production. For instance, increased spending on education improves labour productivity, shifting LRAS to the right. LRAS can be depicted using Classical (perfectly inelastic) or Keynesian (showing spare capacity) models.
c) Full capacity output
Full capacity output represents the maximum level of real GDP an economy can produce when all available factors of production are fully and efficiently employed. In the AD/AS model, this is illustrated by the vertical section of the Keynesian long-run aggregate supply (LRAS) curve or the entirely vertical classical LRAS curve. At this point, the economy is operating on its production possibility frontier. If aggregate demand (AD) increases when the economy is at full capacity, it cannot stimulate further increases in real output; instead, the excess demand purely drives up the general price level, causing demand-pull inflation. To increase full capacity output, the quantity or quality of economic resources must improve, shifting LRAS rightwards.
d) Impact of changes in AD and/or AS on inflation and unemployment
Changes in aggregate demand (AD) and aggregate supply (AS) directly impact an economy's inflation and unemployment rates. An outward shift in AD, assuming upward-sloping AS, increases real output, which reduces cyclical unemployment as firms hire more workers, but it also raises the general price level, causing demand-pull inflation. Conversely, an inward shift in short-run AS, perhaps due to rising raw material costs, reduces real output and raises prices, creating a scenario of stagflation where both unemployment and cost-push inflation increase simultaneously. An outward shift in long-run AS expands the economy's productive capacity, allowing for higher employment and economic growth while putting downward pressure on the price level.
e) The multiplier effect
The multiplier effect occurs when an initial injection into the circular flow of income leads to a proportionately larger final increase in real national income. For example, if the government spends £100 million on infrastructure, construction workers receive this as income. They then spend a portion of it in local businesses, generating further income for shop owners, who in turn spend their earnings. The size of the multiplier depends on the marginal propensity to consume (MPC) and the rate of leakages (saving, taxation, and imports). A higher MPC results in a larger multiplier effect, shifting the aggregate demand (AD) curve further to the right than the initial injection.
f) How the AS/AD model sheds light on the economy as a whole
The AD/AS model is a fundamental macroeconomic tool used to analyse the behaviour of the economy as a whole, illustrating the interaction between total demand and total supply. By plotting the price level against real GDP, the model sheds light on macroeconomic equilibrium and helps explain fluctuations in economic growth, unemployment, and inflation. For instance, a rightward shift in aggregate demand during a boom demonstrates demand-pull inflation and falling cyclical unemployment. Conversely, a leftward shift in short-run aggregate supply illustrates stagflation. The model allows economists to evaluate the potential impacts of fiscal and monetary policies, as well as external shocks, on the broader macroeconomic objectives.
Your focus
- Define Aggregate Demand and state its formula.
- Identify the factors that cause shifts in the Aggregate Demand curve.
- Illustrate changes in Aggregate Demand using appropriate macroeconomic diagrams.
Show all 18 objectives
- Differentiate between the determinants of short-run and long-run aggregate supply.
- Illustrate shifts in the SRAS and LRAS curves using appropriate macroeconomic diagrams.
- Analyse the macroeconomic consequences of changes in aggregate supply on price levels and real GDP.
- Define the concept of full capacity output in macroeconomic terms.
- Illustrate full capacity output using the AD/AS model.
- Analyse the effect of an increase in aggregate demand when an economy is operating at full capacity.
- Analyse the impact of shifts in aggregate demand on the rates of inflation and unemployment.
- Evaluate how changes in short-run aggregate supply affect the price level and real output.
- Illustrate the combined effects of simultaneous shifts in AD and AS on macroeconomic objectives.
- Define the multiplier effect and explain its role in the circular flow of income.
- Calculate the size of the multiplier using data on the marginal propensity to consume or withdraw.
- Analyse how the multiplier effect influences the final shift in aggregate demand and real national income.
- Construct and accurately label an AD/AS diagram to represent the macroeconomic equilibrium.
- Apply the AD/AS model to explain how various shocks and policies affect inflation, unemployment, and economic growth.
- Evaluate the limitations of the AD/AS model in predicting real-world economic outcomes.
The AD/AS model exam tips
Marking Points
- Aggregate Demand (AD) is the total planned expenditure in an economy, calculated as AD = C + I + G + (X - M).
- A shift in the AD curve occurs when there is a change in Consumption, Investment, Government spending, or Net Exports, independent of the price level.
- An increase in AD shifts the curve to the right, which can be caused by factors such as lower interest rates, decreased taxation, or a weaker exchange rate.
- A movement along the AD curve is caused exclusively by a change in the general price level, whereas a shift represents a change in the underlying components.
- A leftward shift in the Short-Run Aggregate Supply (SRAS) curve is caused by an increase in production costs, such as higher wages or raw material prices.
- A rightward shift in the Long-Run Aggregate Supply (LRAS) curve indicates an increase in the economy's productive capacity, driven by improvements in the quantity or quality of factors of production.
- An outward shift in LRAS leads to a lower price level and higher real GDP, assuming Aggregate Demand remains constant.
- Supply shocks, such as a sudden increase in global oil prices, cause an immediate leftward shift in SRAS, leading to cost-push inflation and lower real output.
- Full capacity output occurs when all factors of production are fully utilised, meaning no spare capacity exists in the economy.
- It is represented by the vertical portion of the LRAS curve in the AD/AS model.
- Any increase in aggregate demand when the economy is at full capacity will only result in a higher price level, not higher real GDP.
- An increase in full capacity output requires an outward shift of the LRAS curve, which is achieved through supply-side improvements.
- An outward shift in AD increases real GDP, which lowers unemployment but increases the price level, causing demand-pull inflation.
- An inward shift in AD decreases real GDP, leading to higher cyclical unemployment and a lower price level or disinflation.
- An inward shift in SRAS causes cost-push inflation and reduces real GDP, leading to higher unemployment, a situation known as stagflation.
- An outward shift in LRAS increases real GDP, reducing unemployment while lowering the general price level.
- Define the multiplier effect as the ratio of the final change in income to the initial change in injection.
- Identify the formula for the multiplier, such as 1 / (1 - MPC) or 1 / MPW (where MPW is the marginal propensity to withdraw).
- Explain how an initial injection (investment, government spending, or exports) stimulates further rounds of spending.
- Illustrate the multiplier effect on an AD/AS diagram by showing an initial shift in AD followed by a secondary shift.
- Demonstrate how the intersection of AD and AS determines the equilibrium price level and real national output.
- Use the AD/AS model to explain the causes and consequences of demand-pull and cost-push inflation.
- Apply the model to illustrate changes in unemployment, noting that an increase in real GDP typically corresponds to a fall in cyclical unemployment.
- Evaluate the impact of macroeconomic policies or external shocks by shifting the appropriate AD or AS curves and analysing the new equilibrium.
Examiner Tips
- 💡Always state the AD formula (AD = C + I + G + (X - M)) when explaining a shift in the curve to demonstrate foundational knowledge.
- 💡Clearly identify which specific component of AD is affected by the scenario provided in the stimulus material.
- 💡Use accurately labelled AD/AS diagrams to illustrate the effect of a change in AD on the macroeconomic equilibrium, ensuring arrows show the direction of the shift.
- 💡When analysing supply-side policies, explicitly link them to a rightward shift in the LRAS curve and explain the impact on the macroeconomic equilibrium.
- 💡Use the concept of cost-push inflation to explain the macroeconomic effects of a leftward shift in the SRAS curve, such as during a commodity price shock.
- 💡Ensure your diagrams clearly distinguish between SRAS and LRAS curves, and select the appropriate LRAS model (Keynesian or Classical) based on the context of the question.
- 💡Always draw the LRAS curve vertically at the full capacity level of output when using the classical AD/AS model.
- 💡When evaluating demand-side policies, consider the economy's current position relative to full capacity to determine the likely inflationary impact.
- 💡Use the concept of the production possibility frontier to explain why an economy cannot produce beyond its full capacity output in the long run.
- 💡Draw clear AD/AS diagrams to support your explanations of how shifts affect the macroeconomic objectives of inflation and unemployment.
- 💡Explicitly state whether a shift in AS is short-run (affecting costs) or long-run (affecting productive capacity) when analysing impacts.
- 💡Use the term stagflation when describing the simultaneous increase in inflation and unemployment resulting from an inward shift in SRAS.
- 💡Always state the formula for the multiplier when explaining its impact on the economy.
- 💡Use a step-by-step example of income being spent and re-spent to demonstrate your understanding of the transmission mechanism.
- 💡Evaluate the effectiveness of the multiplier by referencing the current state of the economy, such as the level of spare capacity.
- 💡Draw clear, accurately labelled AD/AS diagrams to support your written analysis in extended responses.
- 💡Explicitly link the shifts in your AD/AS diagram to the core macroeconomic objectives, such as economic growth and price stability.
- 💡Consider the elasticity of the AS curve when evaluating the impact of an AD shift; the effect on the price level versus real output depends heavily on the amount of spare capacity.
Common Mistakes
- Confusing a movement along the AD curve with a shift in the AD curve. Correction: Only changes in the general price level cause a movement; changes in C, I, G, or (X - M) cause a shift.
- Assuming an increase in imports increases AD. Correction: Imports (M) are a leakage from the circular flow, so an increase in imports reduces net exports (X - M) and shifts AD to the left.
- Believing that government spending (G) includes transfer payments. Correction: Transfer payments (like state pensions) are not included in G; they only affect AD when recipients spend the money, which is recorded as Consumption (C).
- Confusing the factors that shift SRAS with those that shift LRAS. Correction: SRAS is shifted by short-term cost changes, whereas LRAS is shifted by long-term changes in productive capacity.
- Assuming an increase in the minimum wage shifts LRAS. Correction: An increase in the minimum wage raises production costs, shifting SRAS to the left, but does not inherently increase the economy's maximum productive capacity.
- Believing that an increase in the price level shifts the AS curve. Correction: A change in the general price level causes a movement along the AS curve, whereas changes in costs or productive capacity cause the curve to shift.
- Confusing full capacity with short-run equilibrium; correction: full capacity is a long-run concept where no further output can be generated without increasing the quantity or quality of resources.
- Assuming an increase in AD at full capacity increases output; correction: at full capacity, an outward shift in AD only causes demand-pull inflation because output cannot expand further.
- Believing full capacity means zero unemployment; correction: full capacity output corresponds to the natural rate of unemployment, which includes frictional and structural unemployment, not zero unemployment.
- Stating that an inward shift in SRAS only causes inflation; correction: an inward shift in SRAS causes both cost-push inflation and a fall in real output, which increases unemployment.
- Assuming AD shifts always cause proportional changes in inflation and unemployment; correction: the impact of an AD shift depends on the elasticity of the AS curve and the amount of spare capacity in the economy.
- Confusing demand-pull and cost-push inflation; correction: demand-pull inflation is caused by outward shifts in AD, whereas cost-push inflation is caused by inward shifts in SRAS.
- Confusing the multiplier effect with the accelerator effect. Correction: The multiplier relates to how injections affect national income, whereas the accelerator relates to how changes in national income affect investment.
- Assuming the multiplier is always a positive whole number. Correction: It can be a fraction if leakages are extremely high, and it can also operate in reverse (a negative multiplier) when injections fall.
- Forgetting that the final impact on real GDP depends on the shape of the AS curve. Correction: If the economy is at full capacity, the multiplier effect will largely result in inflation rather than increased real output.
- Labelling the axes incorrectly. Correction: Students often use microeconomic labels (Price and Quantity) instead of macroeconomic labels (Price Level and Real GDP or National Output).
- Shifting the wrong curve in response to a scenario. Correction: For example, treating a change in income tax as a supply-side shock rather than primarily an aggregate demand shift.
- Failing to distinguish between short-run and long-run aggregate supply when analysing the long-term effects of an economic shock or policy intervention. Correction: Always specify whether the short-run or long-run curve is affected.