3.3 Changes in Equilibrium Price and Quantity: The Four-Step Process

Demand, Supply, and Equilibrium

Market equilibrium is the point where the quantity buyers want to purchase exactly matches the quantity sellers want to produce. This balance sets the price and quantity for a good in the market. When something disrupts that balance, like a change in income, technology, or costs, the equilibrium shifts to a new point. Tracing exactly how that happens is what the four-step process is all about.

The Four-Step Process for Analyzing Equilibrium Changes

This process gives you a systematic way to figure out what happens to price and quantity when market conditions change. You'll use it constantly in this course.

1. Draw the initial demand and supply curves. Start with a standard graph. The demand curve slopes downward (inverse relationship: as price rises, quantity demanded falls). The supply curve slopes upward (direct relationship: as price rises, quantity supplied rises). Label them clearly.

2. Decide whether the event shifts demand, supply, or both. This is the most important step. Ask yourself: does this event change buyers' behavior (demand shift) or sellers' behavior (supply shift)? A change in consumer income shifts demand. A change in production costs shifts supply. Don't confuse a shift of the curve with a movement along it (more on that below).

3. Shift the appropriate curve in the correct direction. An increase shifts the curve to the right. A decrease shifts it to the left. Draw the new curve on your graph.

4. Identify the new equilibrium. Find where the shifted curve intersects the other (unchanged) curve. Read off the new equilibrium price ($P_e$) and equilibrium quantity ($Q_e$). Compare them to the original to determine what changed.

Effects of Demand Shifts

When the demand curve shifts, both equilibrium price and quantity move in the same direction.

• Increase in demand (curve shifts right) → higher equilibrium price and higher equilibrium quantity. For example, population growth means more consumers entering the market, which pushes demand outward.
• Decrease in demand (curve shifts left) → lower equilibrium price and lower equilibrium quantity. A recession that causes widespread job losses reduces consumers' purchasing power, pulling demand inward.

Common demand shifters to remember:

• Consumer income (normal goods vs. inferior goods)
• Population size
• Consumer tastes and preferences
• Prices of related goods (substitutes and complements)
• Expectations about future prices

Effects of Supply Shifts

When the supply curve shifts, equilibrium price and quantity move in opposite directions.

• Increase in supply (curve shifts right) → lower equilibrium price but higher equilibrium quantity. Technological advances like automation reduce production costs, allowing firms to supply more at every price level.
• Decrease in supply (curve shifts left) → higher equilibrium price but lower equilibrium quantity. A natural disaster like a drought destroys crops, reducing the amount producers can bring to market.

Common supply shifters to remember:

• Input prices (raw materials, wages, energy)
• Technology
• Number of producers in the market
• Government policies (taxes, subsidies, regulations)
• Natural events and disruptions

Movements Along a Curve vs. Shifts of a Curve

This distinction trips up a lot of students, so get it straight now.

A movement along a curve happens when the good's own price changes. You're sliding from one point to another on the same curve. If the price of coffee rises and consumers buy less coffee, that's a movement along the demand curve for coffee. Nothing shifted.

A shift of the entire curve happens when something other than the good's own price changes. If a health study makes people want more coffee at every price level, the whole demand curve moves to the right. The curve itself has relocated.

Key Concepts

• Economic equilibrium occurs when quantity demanded equals quantity supplied and there's no pressure for price to change.
• Comparative statics is the method of comparing the old equilibrium to the new one after a shift. That's exactly what the four-step process does.
• The ceteris paribus assumption ("all else equal") lets you isolate the effect of one variable at a time. When you shift only the supply curve, you're holding all demand factors constant.

Real-World Examples

Tech innovation lowers smartphone costs. Advances in 5G chip manufacturing reduce production costs. The supply curve shifts right, leading to a lower equilibrium price and higher equilibrium quantity of smartphones.

Celebrity endorsement boosts sneaker demand. A high-profile athlete endorses a brand, and consumer preferences shift. The demand curve shifts right, leading to a higher equilibrium price and higher equilibrium quantity of sneakers.

Drought reduces crop supply. A severe drought destroys wheat and corn harvests. The supply curve shifts left, leading to a higher equilibrium price and lower equilibrium quantity of agricultural products.

Recession cuts consumer spending. Widespread layoffs reduce household income. The demand curve shifts left, leading to a lower equilibrium price and lower equilibrium quantity across many goods and services.