Reading the Diagram Before Interpreting It
An aggregate demand and aggregate supply diagram can appear simple: output is on one axis, the price level is on the other, and curves meet at an intersection. Errors often arise not from difficult calculations but from assigning the wrong meaning to a curve, a point, or a change between points. A reliable interpretation starts by asking three separate questions: Did a curve shift, or did the economy move along it? What output is actual, and what output is potential? Does the price-level change tell us an inflation rate?
In the standard diagram, real output \(Y\) is on the horizontal axis and the aggregate price level \(P\) is on the vertical axis. The aggregate demand curve (AD) shows combinations of \(P\) and \(Y\) consistent with planned aggregate spending. The short-run aggregate supply curve (SRAS) shows combinations consistent with firms' short-run production decisions. The long-run aggregate supply curve (LRAS) is drawn vertically at potential output, \(Y^*\). These curves describe different relationships; they should not be treated as interchangeable.
The distinction is about what changes in the model, not whether an arrow appears on a diagram. If only the price level changes and the same AD curve is used, the economy moves along AD. If an event changes aggregate demand at each price level, AD shifts. The corresponding distinction applies to SRAS. A price-level change by itself does not establish that either curve shifted.
Movement Along a Curve or a Shift?
To read a diagram, first identify the curve being discussed and then identify the source of the change. A point moving from one location to another on the same AD curve represents a change in real output demanded associated with a changed price level, holding other determinants fixed. By contrast, if the curve itself moves, comparing the old and new curves at the same price level shows a change in aggregate demand.
A similar check applies to SRAS. A change in the price level, with other supply conditions fixed, is represented as movement along SRAS. If a non-price condition of short-run production changes, SRAS shifts. It is a mistake to label every change in the equilibrium price level as a shift of SRAS, or every change in equilibrium output as movement along AD. Equilibrium can change because one or both curves shift; the diagram must be read from the stated cause and the curve positions.
Worked Example: Separating a Price-Level Movement from an AD Shift
Suppose output is measured in millions of units and the price level is in index points. Initially, AD is \(P=110-Y\) and SRAS is \(P=50+Y\). First find short-run equilibrium. Then, on the original AD curve, find the output associated with \(P=70\). Finally, suppose AD shifts to \(P=130-Y\), with SRAS unchanged. Find the new short-run equilibrium and explain the difference between the two changes.
Method. A short-run equilibrium is found where AD and SRAS give the same price level at the same output. Set the two expressions equal:
Rearranging gives \(60=2Y\), so \(Y=30\). Substitution into either curve gives \(P=110-30=80\); checking SRAS gives \(P=50+30=80\). The initial short-run equilibrium is therefore \((Y,P)=(30,80)\).
Now use the original AD curve and set \(P=70\): \(70=110-Y\), so \(Y=40\). This is a movement to a different point along the original AD curve. It is not an equilibrium under the original SRAS: at \(Y=40\), SRAS gives \(P=50+40=90\), not 70.
For the shift, use the new AD equation and the unchanged SRAS:
Thus \(80=2Y\), \(Y=40\), and \(P=50+40=90\). The new equilibrium is \((40,90)\), and both curves give 90 at \(Y=40\). The shift changed the AD relationship; the separate calculation at \(P=70\) merely selected a point on the original AD curve. Common mistake avoided: calling any new point or changed equilibrium a movement along AD without checking whether the AD curve itself changed.
Actual Output Is Not Automatically Potential Output
The AD-SRAS intersection identifies the model's short-run equilibrium: the price level and output at which aggregate demand and short-run aggregate supply are consistent. It does not, by itself, identify potential output. Potential output \(Y^*\) is represented separately by LRAS. Compare the equilibrium output with \(Y^*\) to determine whether the economy is below, at, or above potential.
Potential output is not simply the largest output the economy could ever produce, nor is it automatically the output at every short-run equilibrium. A short-run intersection can lie to the left or right of LRAS. Calling that intersection “potential output” without checking the LRAS position confuses an equilibrium condition with a separate benchmark.
Worked Example: Finding the Output Gap at the Short-Run Intersection
In a hypothetical economy, output is measured in millions of units. AD is \(P=140-Y\), SRAS is \(P=60+Y\), and potential output is \(Y^*=50\). Find the short-run equilibrium, calculate the output gap, and state what the diagram implies about actual output relative to potential. Assume the stated linear curves apply over the relevant output range.
Method. Equate AD and SRAS to find the actual output at the short-run intersection:
Subtract 60 from both sides and add \(Y\) to both sides: \(80=2Y\), so \(Y=40\). The price level is \(P=140-40=100\). Substitution into SRAS checks the result: \(60+40=100\). The short-run equilibrium is \((Y,P)=(40,100)\).
Compare actual output with potential output. The output gap is \(Y-Y^*=40-50=-10\) million units. Actual output is 10 million units below potential. If expressed as a proportion of potential output, the gap is \((-10)/50=-0.20\), or \(-20\%\).
A useful check is to test whether potential output is itself the AD-SRAS intersection. At \(Y=50\), AD implies \(P=140-50=90\), while SRAS implies \(P=60+50=110\). Since these are not equal, \(Y=50\) is not the current short-run equilibrium. Common mistake avoided: reporting 50 as actual output merely because it is the potential-output marker, or reporting 40 as potential merely because it is the intersection.
A Price-Level Change Is Not an Inflation Rate
The vertical coordinate \(P\) is a price level, often represented by a price index. Inflation is a rate of change in the price level over a specified time interval. A diagram that shows a change from one price level to another does not, by itself, state the percentage inflation rate: the starting level and time interval matter.
If a price index is \(P_{t-1}\) in one period and \(P_t\) in the next, the inflation rate over that interval is the change in the index divided by its starting value. This calculation describes the percentage change in the index, not the index value itself.
A higher price level means prices are higher relative to the index's reference base; it does not mean that the inflation rate is higher than before. If the price level continues to rise but rises by a smaller percentage each period, inflation is positive but decelerating. Conversely, a fall in the price level over a period is deflation for that interval. In an AD-AS diagram, a higher equilibrium \(P\) between two outcomes is not enough to calculate inflation unless the relevant periods and price-index values are specified.
Worked Example: Rising Price Level with Slowing Inflation
A hypothetical price index is 150 at the start of Year 1, 156 at the start of Year 2, and 159.12 at the start of Year 3. Calculate the inflation rate in each year-long interval and determine whether the price level is falling in Year 2. Report rates to two decimal places.
Method. For Year 1, the index rises from 150 to 156. Its increase is \(156-150=6\) index points, so the rate is:
For Year 2, the increase is \(159.12-156=3.12\) points. Divide by the starting value for that interval, 156:
The index rises in both intervals, so the price level is not falling in Year 2. Inflation is positive in both years but decreases from 4.00% to 2.00%; this is disinflation. Common mistake avoided: treating 156 as an inflation rate, or calculating Year 2 inflation using 150 rather than the previous period's index of 156.
What the Short-Run Intersection Does—and Does Not—Tell You
The short-run equilibrium intersection is a point satisfying both curves at once. Its output coordinate is the model's short-run actual output, and its vertical coordinate is the corresponding price level. The point is not a prediction of every variable in the economy, nor does it reveal by itself why the curves are where they are. To interpret a change between two intersections, identify which curve or curves shifted and what the starting and ending coordinates are.
The intersection's meaning follows directly from the model's equilibrium condition. At a given output \(Y\), an intersection occurs when the price level implied by AD equals the price level implied by SRAS. If those values differ, that coordinate pair is not on both curves and therefore is not their intersection.
A diagram is a snapshot of modeled relationships, not a time series. Its vertical axis shows the price level, not inflation; its horizontal coordinate at the AD-SRAS intersection is actual short-run output, not necessarily potential output. The intersection alone also does not identify whether a price-level difference arose from a shift in AD, SRAS, or both. Those conclusions require information about the curve changes and the time comparison.
A Practical Reading Sequence
When a question presents a diagram, equations, or a description of an AD-AS change, use a consistent sequence instead of jumping to a conclusion:
Confirm that output is horizontal, the price level is vertical, and distinguish AD, SRAS, and LRAS.
Ask whether a price-level change moves the economy along an unchanged curve, or whether a non-price determinant shifts the curve.
Use the AD-SRAS intersection, or set the two price-level expressions equal when equations are given. Check that both curves give the same price level.
Read actual output from the intersection and compare it with the LRAS coordinate \(Y^*\); do not assume they coincide.
Use the prior period's price level as the denominator and state the interval. A price-level coordinate alone is not an inflation rate.
Common Misreadings to Avoid
- Calling a movement a shift. A changed point on an unchanged AD or SRAS curve is not evidence that the curve shifted. Identify the cause and compare the curve relationships.
- Calling every short-run equilibrium “potential.” Actual output comes from the AD-SRAS intersection; potential output comes from LRAS. Compare them rather than substituting one for the other.
- Calling the price index the inflation rate. An index of 156 is a price-level reading, not inflation of 156%. Calculate the percentage change from the prior level over a stated interval.
- Calling a higher price level “higher inflation” without a time comparison. Inflation depends on the rate of change. A higher level can coexist with a lower inflation rate.
- Treating a point on one curve as an equilibrium. A short-run equilibrium must lie on both AD and SRAS at the same output and price level.
Check Your Understanding
For each item, identify the relevant curve or calculation before deciding what the result means.
- In a hypothetical model, AD is \(P=96-Y\), SRAS is \(P=36+Y\), and LRAS is at \(Y^*=35\). Find the short-run intersection and output gap. (Answer: \(Y=30\), \(P=66\); gap \(=-5\) output units, so actual output is below potential.)
- On an unchanged AD curve \(P=125-Y\), the price level changes from 85 to 75. Find the associated movement in output along AD. Does this fact alone establish an AD shift? (Answer: output changes from 40 to 50, a movement of 10 units along AD; no.)
- A price index rises from 200 to 210 over one year, then rises to 214.2 over the next year. Calculate each annual inflation rate and describe the second year's pattern. (Answer: 5.00% and 2.00%; the price level rises in both years while inflation decelerates.)
- AD is \(P=150-2Y\), SRAS is \(P=30+Y\), and potential output is \(Y^*=45\). Find the short-run equilibrium and output gap. Check whether potential output itself is the short-run intersection. (Answer: equilibrium \(Y=40\), \(P=70\); gap \(=-5\). At \(Y=45\), AD gives \(P=60\) and SRAS gives \(P=75\), so potential output is not the intersection.)
- In an AD-AS diagram, a new point has a higher price level and lower output than the old short-run intersection. No curve positions, cause, or time interval are supplied. State one conclusion that is justified and two that are not justified from those coordinates alone. (Answer: the new point has a higher price level and lower output than the old intersection; one cannot determine which curve shifted or calculate an inflation rate without a time interval and the relevant index values.)