National Income and Price Determination — AP Macroeconomics Macro Study Guide

For: AP Macroeconomics candidates sitting AP Macroeconomics.

Covers: All core AP Macro Unit 3 content including aggregate demand and supply, short-run vs long-run aggregate supply, equilibrium output and price levels, the multiplier effect, and demand-pull and cost-push inflation.

You should already know: No prior econ required.

A note on the practice questions: All worked questions in the "Practice Questions" section below are original problems written by us in the AP Macroeconomics style for educational use. They are not reproductions of past College Board papers and may differ in wording, numerical values, or context. Use them to practise the technique; cross-check with official College Board mark schemes for grading conventions.

1. What Is National Income and Price Determination?

National Income and Price Determination is the core macroeconomic framework that explains how total real output (real GDP) and overall price levels in an economy are set by the interaction of total economy-wide spending and total domestic production. It forms the entire content of AP Macroeconomics Unit 3, accounts for 17-20% of your total exam score per the College Board CED, and is the foundation for all fiscal and monetary policy analysis covered later in the syllabus. It is often referred to interchangeably as the AD-AS (Aggregate Demand-Aggregate Supply) model.

2. Aggregate Demand and Supply

Aggregate Demand (AD) is the total quantity of all goods and services produced in an economy that all economic agents (households, firms, government, and foreign buyers) are willing and able to purchase at each possible price level in a given time period. The formula for AD is identical to the expenditure formula for GDP: $$AD=C+I+G+(X-M)$$ Where $C$ = consumer spending, $I$ = gross private domestic investment, $G$ = government purchases of goods and services, $X$ = export spending by foreigners, and $M$ = import spending on foreign goods. The AD curve slopes downward for three key reasons:

1. Wealth effect: Higher price levels reduce the purchasing power of household savings and assets, so consumer spending falls.
2. Interest rate effect: Higher prices increase demand for money, pushing up interest rates and reducing business investment spending.
3. Exchange rate effect: Higher domestic prices make exports more expensive for foreigners and imports cheaper for domestic buyers, reducing net exports.

Aggregate Supply (AS) is the total quantity of goods and services that firms in an economy are willing and able to produce and sell at each possible price level in a given time period. Unlike microeconomic supply curves that measure single markets, AS measures total economy-wide production.

Worked Example

Suppose an economy has the following expenditure values: $C=\$12T$, $I=\$3T$, $G=\$4T$, $X=\$2T$, $M=\$3T$. Calculate total aggregate demand: $$AD=12+3+4+(2-3)=\$18T$$

3. Short-run vs Long-run AS

The difference between short-run and long-run aggregate supply hinges on the flexibility of input prices (most notably nominal wages for workers):

Short-Run Aggregate Supply (SRAS)

In the short run, nominal wages and other input prices are sticky: they do not adjust immediately to changes in the price level, often due to long-term labor contracts or delayed renegotiation. This makes the SRAS curve upward sloping: when output prices rise but input costs stay fixed, firm profit margins increase, so they raise production levels. SRAS shifts for temporary changes to production costs: changes in commodity prices (e.g. oil, natural gas), changes in nominal wage rates, temporary supply shocks (e.g. natural disasters, pandemic supply chain disruptions), or changes in expected future inflation.

Long-Run Aggregate Supply (LRAS)

In the long run, all input prices are fully flexible: workers and suppliers renegotiate contracts to match changes in the price level, so higher output prices are fully offset by higher input costs. This makes the LRAS curve vertical at the full-employment output level, also called potential GDP ($Y_p$): the output an economy produces when unemployment is at the natural rate (frictional + structural unemployment, no cyclical unemployment). LRAS shifts only for permanent changes to an economy’s production capacity: changes in the size of the labor force, changes in the capital stock (e.g. factories, equipment), improvements in technology, or changes to institutional rules (e.g. patent protection, minimum wage laws).

Worked Example

A country passes a new law expanding access to high-skilled work visas, increasing the size of the labor force by 3% permanently. What happens to SRAS and LRAS? LRAS shifts right, increasing potential GDP by 3%, as the economy can permanently produce more output at every price level. SRAS also shifts right immediately, as the larger labor supply reduces nominal wage costs for firms.

4. Equilibrium Output and Price Level

Short-Run Equilibrium

Short-run macroeconomic equilibrium occurs where the AD curve intersects the SRAS curve. At this point, total quantity of output demanded equals total quantity of output supplied in the short run, so there is no upward or downward pressure on prices. If output is above equilibrium, firms have excess inventory and will cut prices and production; if output is below equilibrium, excess demand pushes prices up and firms raise production.

Long-Run Equilibrium

Long-run macroeconomic equilibrium occurs where AD intersects SRAS and LRAS at the same point. At this equilibrium, the economy is producing at potential GDP, unemployment is at the natural rate, and there is no pressure for prices or wages to adjust in the long run.

Output Gaps

When the economy is in short-run but not long-run equilibrium, it has one of two gaps:

1. Recessionary gap: Short-run equilibrium output is below potential GDP, so cyclical unemployment is present, and price levels are lower than long-run expectations.
2. Inflationary gap: Short-run equilibrium output is above potential GDP, so unemployment is below the natural rate, and price levels are rising as firms compete for limited labor and inputs.

Worked Example

An economy has potential GDP of $\$20T$, and its short-run equilibrium output is $\$22T$ with a price level of 110. What type of gap exists, and what will happen in the long run if no policy action is taken? This is an inflationary gap of $\$2T$, as output is $\$2T$ above potential. In the long run, workers will demand higher nominal wages to match the higher price level, shifting SRAS left until equilibrium returns to the LRAS curve at potential GDP, with a permanently higher price level of 118.

5. Multiplier Effect

The multiplier effect describes how an initial change in any component of aggregate demand leads to a larger final change in total real GDP. This occurs because one person’s spending is another person’s income: when a firm pays a worker for building a new road, that worker spends part of their income on groceries, the grocery store owner spends part of that revenue on new inventory, and so on, creating a chain of additional spending.

First, define two core measures:

• Marginal Propensity to Consume (MPC): The proportion of an additional $\$1$ of disposable income that households spend on consumption: $$MPC=\frac{\Delta C}{\Delta Y_d}$$ Where $\Delta C$ = change in consumption, $\Delta Y_d$ = change in disposable income (income after taxes).
• Marginal Propensity to Save (MPS): The proportion of an additional $\$1$ of disposable income that households save. By definition: $$MPC+MPS=1$$ Every additional dollar of income is either spent or saved.

The spending multiplier is the factor by which an initial change in autonomous spending (e.g. government spending, business investment) is multiplied to get the total change in real GDP: $$SpendingMultiplier=\frac{1}{1-MPC}=\frac{1}{MPS}$$ Total change in real GDP: $$\Delta Y=Multiplier\times \Delta A{D}_{initial}$$

Worked Example

The government increases public transit infrastructure spending by $\$50B$, and the MPC in the economy is 0.8. Calculate the total expected change in real GDP:

1. First calculate the multiplier: $\frac{1}{1-0.8}=5$
2. Total change in GDP: $5\times \$50B=\$250B$

Exam tip: The tax multiplier is smaller than the spending multiplier, because households save a portion of any tax cut instead of spending it. The tax multiplier formula is: $TaxMultiplier=\frac{-MPC}{1-MPC}$, where the negative sign indicates that tax cuts increase GDP, and tax increases reduce GDP.

6. Demand-pull and Cost-push Inflation

Inflation is a sustained increase in the overall price level in an economy over time. There are two core types of inflation, driven by different shifts in the AD-AS model:

Demand-pull Inflation

Demand-pull inflation occurs when an increase in aggregate demand pushes output above potential GDP, leading to higher prices as firms compete for limited inputs. It is often described as "too much money chasing too few goods". Common causes include: cuts to personal or corporate taxes, increases in government spending, lower interest rates that boost consumer and business spending, or a boom in foreign demand for domestic exports. When AD shifts right, short-run output rises above potential, unemployment falls below the natural rate, and price levels rise. In the long run, SRAS shifts left to adjust to higher wages, returning output to potential but leaving prices permanently higher.

Cost-push Inflation

Cost-push inflation occurs when a negative supply shock increases input costs for firms, shifting the SRAS curve left, leading to higher prices and lower output (a state called stagflation). Common causes include: large increases in oil or commodity prices, unexpected rises in nominal wages, natural disasters that destroy production capacity, or global supply chain disruptions. Cost-push inflation is far harder for policymakers to address: shifting AD right will reduce unemployment but make inflation worse, while shifting AD left will reduce inflation but raise unemployment further.

Worked Example

In 2021, the US government passed a $\$1.9T$ stimulus bill during a period when the economy was already approaching potential GDP, while global supply chain disruptions increased input costs for manufacturing firms. Identify the types of inflation occurring: The stimulus bill increased consumer and government spending, shifting AD right and causing demand-pull inflation. The supply chain disruptions increased input costs, shifting SRAS left and causing cost-push inflation. The combination of both shocks led to the 9% peak inflation recorded in the US in 2022.

7. Common Pitfalls (and how to avoid them)

• Pitfall 1: Drawing AD as upward sloping or SRAS as downward sloping. Why it happens: Confusion between microeconomic supply/demand curves and aggregate AD/AS curves. Correct move: Always label your curves clearly on graphs, and remember the three effects that make AD downward sloping, and sticky input prices that make SRAS upward sloping. Examiners deduct 1-2 marks for mislabeled curves on FRQs.
• Pitfall 2: Calculating the spending multiplier as $\frac{1}{MPC}$ instead of $\frac{1}{1-MPC}$. Why it happens: Mixing up MPC and MPS in the formula. Correct move: Always write $MPC+MPS=1$ first before calculating the multiplier, so you remember to use MPS in the denominator. For MPC = 0.8, MPS = 0.2, multiplier = 5, not 1.25.
• Pitfall 3: Shifting LRAS for temporary supply shocks (e.g. a 6-month increase in oil prices). Why it happens: Assuming all supply shocks affect long-run production capacity. Correct move: Only permanent changes to labor, capital, technology, or institutions shift LRAS. Temporary shocks only shift SRAS, and LRAS remains at potential GDP.
• Pitfall 4: Assuming inflationary gaps are sustainable in the long run. Why it happens: Assuming firms can keep producing above potential forever without rising input costs. Correct move: In the long run, nominal wages adjust upward to match higher price levels, so SRAS shifts left, returning output to potential GDP automatically over 1-3 years, unless policymakers intervene first.
• Pitfall 5: Using the spending multiplier for tax change questions. Why it happens: Forgetting that tax cuts are partially saved, so their effect is smaller. Correct move: Use the tax multiplier ($\frac{-MPC}{1-MPC}$) for all tax change questions, and the spending multiplier for changes to government spending, investment, or net exports.

8. Practice Questions (AP Macroeconomics Style)

Question 1

An economy has a marginal propensity to consume of 0.75, and is currently facing a recessionary gap of $\$250B$. The government implements a personal income tax cut of $\$100B$, with no crowding out effect. a) Calculate the tax multiplier for this economy. b) Calculate the total expected change in real GDP from this tax cut. c) Will this tax cut close the recessionary gap fully? Explain your answer.

Solution

a) Tax multiplier formula: $\frac{-MPC}{1-MPC}=\frac{-0.75}{1-0.75}=-3$. The negative sign indicates that tax cuts increase GDP. b) Total change in GDP: $\Delta Y=-3\times (-\$100B)=\$300B$ c) No, the tax cut will not close the gap fully. The recessionary gap is $\$250B$, but the tax cut increases GDP by $\$300B$, leading to a new $\$50B$ inflationary gap.

Question 2

For each of the following events, identify the type of inflation caused, and the corresponding shift in the AD-AS model: a) A 25% rise in global natural gas prices, which are a key input for manufacturing and electricity generation. b) A 15% increase in household wealth from a stock market boom, during a period of full employment. c) A central bank cuts interest rates by 2 percentage points when the economy is already at potential GDP.

Solution

a) Cost-push inflation: Higher natural gas prices increase input costs for firms, shifting SRAS left, leading to higher prices and lower output. b) Demand-pull inflation: Higher household wealth increases consumer spending, shifting AD right, leading to higher prices and output above potential. c) Demand-pull inflation: Lower interest rates boost consumer and business investment spending, shifting AD right, leading to higher prices as output cannot rise above potential in the long run.

Question 3

An economy is in long-run equilibrium. A severe drought permanently destroys 15% of the country’s agricultural production capacity. a) What happens to SRAS, LRAS, and AD in the short run? b) What is the short-run effect on output, price level, and unemployment? c) If no policy action is taken, what happens in the long run to return the economy to equilibrium?

Solution

a) SRAS shifts left immediately due to lost agricultural production, LRAS also shifts left because the loss of production capacity is permanent, reducing potential GDP. AD remains unchanged in the short run. b) Short-run output falls, price level rises, and unemployment rises above the natural rate (stagflation). c) In the long run, nominal wages adjust downward to match the lower output level, so SRAS shifts slightly right until the new short-run equilibrium intersects the new, lower LRAS curve, at a higher price level and lower output than the original long-run equilibrium.

9. Quick Reference Cheatsheet

Core Formulas

Key Rules

• AD is downward sloping (wealth, interest rate, exchange rate effects)
• SRAS is upward sloping (sticky input prices in the short run)
• LRAS is vertical at potential GDP $Y_p$ (flexible input prices in the long run)
• Short-run equilibrium = $AD\cap SRAS$; Long-run equilibrium = $AD\cap SRAS\cap LRAS$
• Recessionary gap: $Y_{eq}<Y_p$; Inflationary gap: $Y_{eq}>Y_p$
• Demand-pull inflation = right shift in AD; Cost-push inflation = left shift in SRAS

10. What's Next

This AD-AS framework is the foundation for all remaining content in the AP Macroeconomics syllabus. You will use it to analyze the effects of fiscal policy (government spending and tax changes, covered in Unit 4) and monetary policy (central bank interest rate and money supply adjustments, covered in Unit 5), as well as long-run economic growth and open-economy trade and exchange rate effects (Unit 6). Calculating the multiplier to determine the required size of policy interventions to close output gaps is one of the most frequently tested skills on both the multiple-choice and free-response sections of the AP exam, so mastering this topic is critical for a top score.

If you have questions about any of the concepts, formulas, or practice questions in this guide, you can ask Ollie, our AI tutor, for personalized explanations, additional practice problems, or step-by-step walkthroughs of AD-AS graphing questions at any time on Ollie. Be sure to test your knowledge with official College Board past papers to get comfortable with the exam’s question format and grading rubrics before your test date.