Class 11 Micro Economics Notes · CBSE

Simultaneous Changes in Demand and Supply

Simultaneous Changes in Demand and Supply — understanding how the relative magnitude of demand and supply shifts determines the new equilibrium. CBSE Class 11 Microeconomics notes with twelve cases and graphs.

Last updated: 16 Sep 2026

Notes

Why Magnitude Matters

Situations which lead to simultaneous changes in both demand and supply. To predict whether the equilibrium price and the equilibrium quantity rise or fall in such cases, we need to know the magnitude of changes in both demand and supply.

The Demand and Supply model is very easy to use when there is a change in either demand or supply. However, in reality, there are a number of situations which lead to simultaneous changes in both demand and supply.

There are 4 cases of simultaneous shifts in demand and supply curves:

(I) Both Demand and Supply decrease

(II) Both Demand and Supply increase

(III) Demand decreases and Supply increases

(IV) Demand increases and Supply decreases

The outcome depends on the relative magnitude of the two shifts: when both curves move in the same direction, the quantity outcome is definite but the price outcome is ambiguous; when they move in opposite directions, the price outcome is definite but the quantity outcome is ambiguous.

Case Explorer — The Twelve Cases

When the decrease in demand is proportionately equal to the decrease in supply, then the leftward shift in the demand curve from DD to D₁D₁ is proportionately equal to the leftward shift in the supply curve from SS to S₁S₁.

New equilibrium is determined at E₁. As demand and supply decrease in the same proportion, the equilibrium price remains the same at OP, but the equilibrium quantity falls from OQ to OQ₁.

When the decrease in demand is proportionately more than the decrease in supply, then the leftward shift in the demand curve from DD to D₁D₁ is proportionately more than the leftward shift in the supply curve from SS to S₁S₁.

The new equilibrium is determined at E₁, the equilibrium price falls from OP to OP₁ and the equilibrium quantity falls from OQ to OQ₁.

When the decrease in demand is proportionately less than the decrease in supply, then the leftward shift in the demand curve from DD to D₁D₁ is proportionately less than the leftward shift in the supply curve from SS to S₁S₁.

The new equilibrium is determined at E₁, the equilibrium price rises from OP to OP₁ whereas the equilibrium quantity falls from OQ to OQ₁.

The Outcome Matrix

Outcome Matrix: Simultaneous Changes in Demand and Supply
Scenario	Equilibrium Price	Equilibrium Quantity
Both demand and supply increase	Ambiguous — rises if ΔD > ΔS; falls if ΔD < ΔS; same if equal	Definitely rises
Both demand and supply decrease	Ambiguous — falls if ΔD > ΔS; rises if ΔD < ΔS; same if equal	Definitely falls
Demand increases and supply decreases	Definitely rises	Ambiguous — rises if ΔD > ΔS; falls if ΔD < ΔS; same if equal
Demand decreases and supply increases	Definitely falls	Ambiguous — falls if ΔD > ΔS; rises if ΔD < ΔS; same if equal

Test Yourself

A new health campaign boosts demand for wheat while drought reduces the wheat crop (supply falls).

Farm incomes rise and better seeds raise wheat output; the increase in supply is proportionately more than the increase in demand.

Demand for tea falls proportionately more than supply of tea falls.

Demand for cars increases proportionately more than supply of cars increases.

Demand for milk falls but supply of milk rises proportionately less than the fall in demand.

Demand for sugar rises while supply of sugar decreases proportionately more.

Key Takeaways

When demand and supply shift together, the outcome depends on the relative magnitude of the two shifts — never assume without comparing. ⭐
Both increase → quantity definitely rises; price is ambiguous (same if shifts are equal, rises if ΔD > ΔS, falls if ΔD < ΔS). ⭐
Both decrease → quantity definitely falls; price is ambiguous (same if equal, falls if ΔD > ΔS, rises if ΔD < ΔS). ⭐
Demand increases and supply decreases → price definitely rises; quantity is ambiguous. ⭐
Demand decreases and supply increases → price definitely falls; quantity is ambiguous. ⭐
In all twelve cases the new equilibrium E₁ is found at the intersection of the new demand and new supply curves; the old price only starts the adjustment. ⭐