AP Macroeconomics: AD?AS Model and Macroeconomic Equilibrium

AP Macroeconomics – AD?AS Model and Macroeconomic Equilibrium

What This Is

The Aggregate Demand–Aggregate Supply (AD?AS) model shows how the total quantity of goods and services demanded (AD) and supplied (AS) in an economy interact to determine overall output (real GDP) and the price level. It is a cornerstone of the AP?Macroeconomics exam because every FRQ that asks about inflation, unemployment, fiscal or monetary policy starts with an AD?AS diagram. Real?world example: When the Federal Reserve cuts the federal funds rate during a recession, the lower cost of borrowing shifts AD rightward, raising output and (usually) the price level.

Key Terms & Formulas

• Aggregate Demand (AD) curve – Downward?sloping; P (price level) on the vertical axis, Y (real GDP) on the horizontal axis. Shows all combinations of P and Y where the total planned spending equals output.
• Short?Run Aggregate Supply (SRAS) curve – Upward?sloping in the short run; same axes as AD. Reflects that firms will produce more when prices rise while some input costs are still sticky.
• Long?Run Aggregate Supply (LRAS) curve – Vertical at Y* (potential GDP). In the long run, output is determined solely by resources, technology, and institutions, not by the price level.
• Potential GDP (Y*) – The level of real output the economy can sustain at full employment; equals the LRAS intercept.
• Recessionary Gap – The distance between Y* and the equilibrium output when AD is left of LRAS (output below potential).
• Inflationary Gap – The distance between equilibrium output and Y* when AD is right of LRAS (output above potential).
• Fiscal Policy Shift of AD – ?AD = (1?MPC)·?G + MPC·?T (where MPC = marginal propensity to consume, ?G = change in government spending, ?T = change in taxes). Positive ?G or negative ?T shifts AD right; opposite shifts left.
• Monetary Policy Shift of AD – ?AD = k·?M (k = money multiplier, ?M = change in the money supply). An expansionary policy (lower discount rate, open?market purchase) shifts AD right; contractionary does the opposite.
• Phillips Curve (SR) – Inverse relation between inflation (?) and unemployment (U). Axes:? (vertical) vs. U (horizontal). Shows the short?run trade?off that can be illustrated with AD?AS movements.
• Price Level (P) – Measured by a broad price index (CPI or GDP?deflator). In AD?AS, a movement along the AD curve changes P while output stays fixed; a shift of AD changes both P and Y.

Step?by?Step / Process Flow

1. Draw the baseline AD?AS diagram – Plot LRAS vertical at Y*, SRAS upward, and AD downward. Label the equilibrium (E?) with price level P? and output Y?.
2. Identify the shock – e.g., a tax increase on consumer goods (contractionary fiscal policy). Decide which curve shifts: AD leftward for lower consumer spending.
3. Shift the curve – Move the AD curve left to AD?. Keep SRAS and LRAS unchanged.
4. Find the new equilibrium – Intersection of AD? and SRAS gives new price level P? (lower) and output Y? (lower).
5. Interpret the gap – Compare Y? to Y*: if Y<?Y*, a recessionary gap exists; discuss likely unemployment rise and policy options.
6. Link to policy – Explain how an expansionary monetary policy (e.g., Fed lowers discount rate) would shift AD rightward back toward LRAS, closing the gap.

Common Mistakes

• Mistake: Treating a movement along the SRAS curve as a “shift” in supply.
  Correction: A shift of SRAS occurs when input prices or technology change; a movement along SRAS is caused by a change in the price level while other factors stay constant.

• Mistake: Confusing a change in AD (curve shift) with a change in quantity demanded (movement along AD).
  Correction: Only factors that affect all components of aggregate spending (e.g., fiscal policy, consumer confidence) shift AD; a change in the price level moves the economy along the existing AD curve.

• Mistake: Saying “inflationary gap means prices must rise” without noting that output also rises above potential.
  Correction: An inflationary gap raises both the price level and real GDP above Y*, creating upward pressure on wages and future inflation.

• Mistake: Forgetting that LRAS is vertical in the short run only when prices are fully flexible; in the very short run (sticky wages) LRAS can be upward?sloping.
  Correction: For AP purposes, treat LRAS as vertical; any deviation from Y* is a short?run phenomenon.

• Mistake: Using nominal GDP instead of real GDP when measuring output on the AD?AS graph.
  Correction: AD?AS always plots real GDP (output adjusted for price changes) on the horizontal axis; the price level is shown separately on the vertical axis.

AP Exam Insights

1. FRQ Prompt Pattern: “Explain how a contractionary fiscal policy will affect the AD?AS diagram and the economy’s unemployment rate.” You must (a) identify the AD shift, (b) show the new equilibrium, (c) label the recessionary gap, and (d) discuss the unemployment implication.
2. Distinguish Short?Run vs Long?Run: Many items ask you to state whether a policy “affects only SRAS” or “shifts LRAS.” Remember: only changes in resources, technology, or institutional factors move LRAS.
3. Multiple?Choice Trap: A question may give a rise in the price level and ask which curve moved. The correct answer is SRAS (movement along AD) if the output stays the same, not AD.
4. Graphing Requirement: You will be asked to draw a “complete” AD?AS diagram, including labels for P?, Y?, LRAS, and the gap. Leave space for a brief explanation; the AP rubric awards points for both the diagram and the written analysis.

Quick Check Questions

1. MC: A 10?% increase in the federal funds rate is most likely to cause which of the following in the AD?AS model?
2. A) SRAS shifts left
3. B) AD shifts left
4. C) LRAS shifts right

5. D) AD shifts right
   Answer: B) AD shifts left – higher rates reduce borrowing, cutting consumption and investment.

6. FRQ?style: “The government raises the corporate tax rate. Using the AD?AS model, describe the short?run effect on real GDP and the price level, and state one likely impact on unemployment.”
   Answer: AD shifts left, moving equilibrium to a lower price level (P?) and lower real GDP (Y?); the economy experiences a recessionary gap, so unemployment rises.

7. MC: Which of the following would shift LRAS to the right?

8. A) A temporary increase in oil prices
9. B) A permanent improvement in technology
10. C) A rise in consumer confidence
11. D) A decrease in the federal budget deficit
    Answer: B) A permanent improvement in technology – it expands the economy’s productive capacity.

Last?Minute Cram Sheet

1. AD curve: Downward; P vs. Y.
2. SRAS curve: Upward in short run; sticky input prices.
3. LRAS curve: Vertical at potential GDP (Y*).
4. Recessionary gap: AD left of LRAS-output below potential.
5. Inflationary gap: AD right of LRAS-output above potential.
6. Fiscal expansion: ?G or ?T-AD right.
7. Monetary expansion: ?discount rate / open?market purchase-?M-AD right.
8. ?AD = (1?MPC)·?G + MPC·?T – remember sign convention.
9. “Supply increases” = curve shifts right, not up; price?level change is a movement along the curve.
10. Phillips Curve (SR):-? when U ?; illustrated by AD?AS shifts.