Welcome to Aggregate Supply (AS)
Welcome to one of the most fundamental topics in macroeconomic analysis! If you have ever wondered how the total output of an entire nation is determined and how businesses across the country react when the general price level changes, you are in the right place.
Don't worry if macroeconomics feels a little abstract compared to microeconomics. While microeconomics looks at single markets (like the market for coffee or smartphones), Aggregate Supply simply zooms out to look at all producers across the whole economy combined together. By the end of this guide, you will master the definition of Aggregate Supply, understand how to draw and interpret the AS curve, distinguish between shifts and movements, and clearly explain the critical distinction between the short run and the long run.
1. What is Aggregate Supply (AS)?
In economics, the word "aggregate" simply means "total" or "added together".
Official Definition: Aggregate Supply (AS) is the total volume and value of all final goods and services that producers in an economy are willing and able to supply at any given general price level over a specified period of time.
Think of it as the grand total of everything made in a country—from cars and haircuts to software and healthcare—ready for sale at a particular price level.
2. The Short-Run Aggregate Supply (SRAS) Curve
When we represent Aggregate Supply on a macroeconomic diagram, we plot the relationship between the nation's overall price level and the total amount of goods and services produced.
Diagrammatic Conventions: Getting the Axes Right
Examiners frequently spot students losing easy marks by using microeconomic labels. Always label your macroeconomic axes correctly:
• Vertical (\(y\)) Axis: General Price Level (or Average Price Level / CPI / GDP Deflator). Never label this just "Price" or "\(P\)".
• Horizontal (\(x\)) Axis: Real National Output (or Real GDP / Output / \(Y\)). Never label this just "Quantity" or "\(Q\)".
The Shape of the SRAS Curve
The Short-Run Aggregate Supply (SRAS) curve slopes upwards from left to right. This shows a positive relationship between the general price level and the level of real national output: as the general price level rises, firms are willing to supply more output.
3. Why Does the SRAS Curve Slope Upwards?
Why do producers across the entire economy increase their output when the price level rises in the short run? There are three main economic explanations you must understand:
1. The Microeconomic Basis:
Macroeconomic aggregate supply is the aggregation (sum total) of all individual firms' supply curves in the economy. Because individual supply curves slope upwards due to the profit motive, the economy-wide supply curve also slopes upwards.
2. Short-Run Diminishing Returns and Rising Marginal Costs:
In the short run, at least one factor of production is fixed (such as capital equipment or factory floor space). To increase output quickly, firms must use their existing fixed capital more intensively—for example, by paying workers overtime rates or dealing with factory bottlenecks. This increases the marginal cost (the cost of producing one extra unit). Firms will only produce this extra output if the price level rises enough to cover these higher unit costs.
3. Sticky Wages and Sticky Input Prices:
In the short run, many production costs are fixed or slow to change because wages are tied to annual employment contracts and raw materials are purchased under fixed-price contracts. This phenomenon is known as "sticky wages" or sticky costs. When the general price level rises, the prices firms receive for their final goods increase while their production costs stay temporarily fixed. This expands their profit margins per unit, providing a strong incentive for firms to hire more workers, expand output, and produce higher volumes.
Analogy Time: The Bakery on Overtime
Imagine a bakery with two fixed ovens. In the short run, the baker cannot instantly build a larger bakery. If bread prices rise in the market, the baker wants to sell more bread to make higher profits. To bake more, they have to pay their staff expensive weekend overtime and run the ovens at maximum capacity. Because producing those extra loaves costs more per loaf, the baker is only willing to expand production if the selling price is high enough to make it worthwhile.
Key Takeaway: The SRAS curve slopes upwards because higher prices compensate firms for rising short-run marginal costs and create temporary profit incentives while nominal wages remain fixed.
4. Movements Along vs Shifts of the AS Curve
Just like in microeconomics, distinguishing between a movement along a curve and a shift of the curve is an essential exam skill.
Movements Along the AS Curve
A movement along the aggregate supply curve is caused strictly and exclusively by a change in the general price level (which is usually triggered by a shift in Aggregate Demand).
• Extension (Expansion) in AS: Occurs when the general price level rises, leading to an increase in real national output along the existing curve.
• Contraction in AS: Occurs when the general price level falls, leading to a decrease in real national output along the existing curve.
Shifts of the AS Curve
A shift of the AS curve occurs when there is a change in the non-price conditions of supply—primarily changes in the costs of production or productive capacity at any given price level.
• Rightward Shift (\(\text{AS}_1 \to \text{AS}_2\)): Represents an increase in aggregate supply. Producers are willing to supply more output at every price level (e.g., due to lower production costs or higher labour productivity).
• Leftward Shift (\(\text{AS}_1 \to \text{AS}_3\)): Represents a decrease in aggregate supply (also known as a negative supply shock). Producers supply less output at every price level (e.g., due to a surge in raw material costs, higher wage rates, or an increase in indirect business taxes).
Quick Memory Aid: The "P" Rule
• Price level change = Point moves along the curve.
• Cost / non-price change = Curve shifts entirely.
5. The Relationship Between Short-Run AS (SRAS) and Long-Run AS (LRAS)
Understanding the transition between the short run and the long run is crucial for achieving top marks in A Level Economics.
Defining the Time Horizons
In economics, the distinction between the short run and the long run is not measured in calendar days, weeks, or months. It is defined by whether factor prices have had time to adjust:
• Short Run (SRAS): The period of time in which the prices of factors of production are fixed. In particular, nominal wage rates and raw material supply contracts are "sticky" and do not immediately change when the general price level changes.
• Long Run (LRAS): The period of time in which all factor prices are flexible and have fully adjusted to changes in the price level. In the long run, the economy operates at its full-capacity potential output.
How the Economy Transitions from SRAS to LRAS
What happens when an economy produces above or below its normal capacity?
1. If aggregate demand surges, firms push output beyond normal capacity along the SRAS curve by using existing machinery intensely and paying overtime.
2. However, this high level of output causes shortages of labour and raw materials.
3. Over time, workers demand higher nominal wages to match the higher cost of living, and suppliers raise raw material prices.
4. As these factor prices adjust upwards, the costs of production rise across the entire economy.
5. This rise in production costs shifts the SRAS curve to the left, moving the economy back towards its sustainable long-run capacity on the LRAS curve.
Key Takeaway: The short run is characterised by fixed factor prices (sticky wages), whereas the long run represents the state of the economy once all wages and factor prices have completely adjusted.
6. Common Pitfalls & Examiner Warnings
Avoid these frequent exam errors highlighted in Pearson Edexcel examiner reports:
• Micro vs Macro Axis Errors: Never label the axes "\(P\)" and "\(Q\)". Always write Price Level and Real GDP (or Real National Output / \(Y\)).
• Confusing Shifts with Movements: A change in the general price level does not shift the SRAS curve. It causes an extension or contraction along the existing SRAS curve.
• Confusing SRAS Shifts with LRAS Shifts: Temporary variable cost changes (such as a temporary spike in oil prices or a short-term currency fluctuation) shift the SRAS curve, not the LRAS curve. Permanent changes to the productive capacity of the economy (like education or technological advancement) shift the LRAS curve.
• Incorrect Definition of the Short Run: Do not define the short run as "less than a year". Always define it economically: a period when at least one factor input or nominal wage rate is fixed.
Quick Review Summary
• Aggregate Supply (AS): Total final output produced across the entire economy at a given price level over a period of time.
• Axes: Price Level (vertical) vs Real National Output / Real GDP / \(Y\) (horizontal).
• SRAS Shape: Upward-sloping due to microeconomic aggregation, rising marginal costs (diminishing returns), and sticky nominal wages boosting profit margins.
• Price Level Changes: Cause movements along the curve (extensions or contractions).
• Cost / Supply Factor Changes: Shift the curve left (decrease/shock) or right (increase).
• Short Run vs Long Run: The short run has fixed factor prices (sticky wages); the long run is reached when all factor prices have fully adjusted to equilibrium.