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๐Ÿ“ˆ Economics ยท Supply & Demand

Memory tricks for supply and demand

Demand curves, supply shifters, equilibrium, elasticity, consumer surplus, price controls, and market efficiency โ€” the foundation of economics.

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๐Ÿ“ˆ Supply & Demand

Memory Tricks

Proven Mnemonics & Acronyms โ€” fast to learn, hard to forget.

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Demand Curve
Demand slopes DOWN โ€” Price up, Quantity down
Inverse relationship between price and quantity demanded
Inverse relationship between price and quantity demanded
Law of demand: higher prices reduce quantity demanded. Substitution effect (good becomes more expensive vs alternatives) and income effect (less purchasing power). Curve shows what consumers are WILLING and ABLE to buy at each price.
Movement vs shift
Movement ALONG: caused by price change only. Shift OF: caused by non-price factors. Critical distinction.
Individual vs market demand
Market demand = horizontal sum of all individual demands. At each price, add all quantities.
Ceteris paribus
All else equal. Non-price factor changes shift the entire curve.
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๐Ÿƒ Demand Curve
Why does demand slope down?
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๐Ÿƒ Answer
Demand slopes DOWN โ€” Price up, Quantity down
Movement vs shiftMovement ALONG: caused by price change only. Shift OF: caused by non-price factors. Critical distinction.
Individual vs market demandMarket demand = horizontal sum of all individual demands. At each price, add all quantities.
Ceteris paribusAll else equal. Non-price factor changes shift the entire curve.
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Demand Shifters
SPENT (S=Substitutes, P=Preferences, E=Expectations, N=Number of buyers, T=income Type) โ€” the five demand shifters: Substitutes, Preferences, Expectations, Number of buyers, Tastes/Income
Five factors that shift the entire demand curve
Five factors that shift the entire demand curve
Demand shifts with: Substitute prices (Pepsi rises, Coke demand rises). Preferences. Expectations (expect price rise, buy now). Number of buyers. Income (normal goods: more income = more demand; inferior goods: opposite).
Normal vs inferior goods
Normal: demand rises with income. Inferior: demand falls with income. Most goods are normal.
Substitute goods
Price of substitute rises, demand for original increases. Positive cross-price elasticity.
Complement goods
Price of complement rises, demand for original falls. Negative cross-price elasticity.
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๐Ÿƒ Demand Shifters
SPENT
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๐Ÿƒ Answer
SPENT (S=Substitutes, P=Preferences, E=Expectations, N=Number of buyers, T=income Type) โ€” the five demand shifters: Substitutes, Preferences, Expectations, Number of buyers, Tastes/Income
Normal vs inferior goodsNormal: demand rises with income. Inferior: demand falls with income. Most goods are normal.
Substitute goodsPrice of substitute rises, demand for original increases. Positive cross-price elasticity.
Complement goodsPrice of complement rises, demand for original falls. Negative cross-price elasticity.
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Supply Curve
Supply slopes UP โ€” Price up, Quantity up
Direct relationship between price and quantity supplied
Direct relationship between price and quantity supplied
Law of supply: higher prices increase quantity supplied. Higher prices cover higher marginal costs. The supply curve shows what producers are WILLING and ABLE to sell at each price.
Why upward sloping
Increasing marginal costs as firms expand. Higher prices attract new producers.
Producer surplus
Area above supply curve, below price. Price increase raises producer surplus.
Short vs long run supply
Long-run more elastic: more time to adjust capacity, enter or exit market.
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๐Ÿƒ Supply Curve
Why does supply slope up?
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๐Ÿƒ Answer
Supply slopes UP โ€” Price up, Quantity up
Why upward slopingIncreasing marginal costs as firms expand. Higher prices attract new producers.
Producer surplusArea above supply curve, below price. Price increase raises producer surplus.
Short vs long run supplyLong-run more elastic: more time to adjust capacity, enter or exit market.
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Supply Shifters
ROTTEN (R=Resources, O=Other goods, T=Technology, T=Taxes/subsidies, E=Expectations, N=Number of sellers) โ€” the six supply shifters: Resources, Other goods, Technology, Taxes/subsidies, Expectations, Number of sellers
Six factors that shift the entire supply curve
Six factors that shift the entire supply curve
Supply shifts with: Resource/input costs (wages rise = supply left). Other goods prices. Technology (improvement = supply right). Taxes (supply left) / Subsidies (supply right). Expectations. Number of sellers.
Input costs
Rising wages or materials shift supply left โ€” higher costs reduce profit at each price.
Technology
Reduces production cost โ€” supply increases (right shift). Explains falling technology prices.
Taxes vs subsidies
Per-unit tax: supply left (acts like cost increase). Subsidy: supply right (reduces effective cost).
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๐Ÿƒ Supply Shifters
ROTTEN
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๐Ÿƒ Answer
ROTTEN (R=Resources, O=Other goods, T=Technology, T=Taxes/subsidies, E=Expectations, N=Number of sellers) โ€” the six supply shifters: Resources, Other goods, Technology, Taxes/subsidies, Expectations, Number of sellers
Input costsRising wages or materials shift supply left โ€” higher costs reduce profit at each price.
TechnologyReduces production cost โ€” supply increases (right shift). Explains falling technology prices.
Taxes vs subsidiesPer-unit tax: supply left (acts like cost increase). Subsidy: supply right (reduces effective cost).
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Equilibrium
P* where Qd = Qs โ€” no shortage, no surplus, market clears
Equilibrium is stable โ€” deviations create self-correcting forces
Equilibrium is stable โ€” deviations create self-correcting forces
Equilibrium: quantity demanded = quantity supplied. Self-correcting: surplus โ†’ price falls. Shortage โ†’ price rises. Equilibrium changes when supply or demand shifts.
Finding equilibrium
Set Qd = Qs, solve for price. Substitute back for quantity.
Comparative statics
Demand right shift: P and Q both rise. Supply right shift: P falls, Q rises.
Speed of adjustment
Financial markets: seconds. Labor markets: months. Housing: years.
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๐Ÿƒ Equilibrium
Equilibrium price โ€” what happens there?
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๐Ÿƒ Answer
P* where Qd = Qs โ€” no shortage, no surplus, market clears
Finding equilibriumSet Qd = Qs, solve for price. Substitute back for quantity.
Comparative staticsDemand right shift: P and Q both rise. Supply right shift: P falls, Q rises.
Speed of adjustmentFinancial markets: seconds. Labor markets: months. Housing: years.
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Price Elasticity
PED (Price Elasticity of Demand) = %ฮ”Qd / %ฮ”P โ€” elastic > 1, inelastic < 1
Elasticity measures how much quantity responds to price changes
Elasticity measures how much quantity responds to price changes
Elastic (>1): quantity changes more than price โ€” luxuries, goods with substitutes. Inelastic (<1): quantity changes less โ€” necessities, few substitutes, addictive goods. Unit elastic (=1): equal changes.
Total revenue test
Elastic: price and revenue move opposite. Inelastic: price and revenue move same direction. Unit elastic: no revenue effect.
Perfect cases
Perfectly elastic: horizontal demand (any price rise loses all customers). Perfectly inelastic: vertical demand (quantity never changes).
Midpoint method
Use average of two points as base to avoid different answers from same data depending on direction.
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๐Ÿƒ Price Elasticity
PED
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๐Ÿƒ Answer
PED (Price Elasticity of Demand) = %ฮ”Qd / %ฮ”P โ€” elastic > 1, inelastic < 1
Total revenue testElastic: price and revenue move opposite. Inelastic: price and revenue move same direction. Unit elastic: no revenue effect.
Perfect casesPerfectly elastic: horizontal demand (any price rise loses all customers). Perfectly inelastic: vertical demand (quantity never changes).
Midpoint methodUse average of two points as base to avoid different answers from same data depending on direction.
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Consumer & Producer Surplus
CS (Consumer Surplus) + PS (Producer Surplus) = Total Surplus โ€” maximized at free market equilibrium
Surplus measures gains from trade for buyers and sellers
Surplus measures gains from trade for buyers and sellers
Consumer Surplus = area below demand, above price. Producer Surplus = area above supply, below price. Total Surplus = CS + PS = total gains from trade. Free market maximizes total surplus. Deviations create deadweight loss.
Deadweight loss
Loss of total surplus from non-equilibrium. Taxes, price controls, monopoly all create DWL.
Tax incidence
More inelastic side bears more of tax burden. Perfectly inelastic demand: buyers bear 100%.
Price ceiling effects
Ceiling below equilibrium: shortage. Total surplus decreases (DWL created). CS may rise or fall.
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๐Ÿƒ Consumer & Producer Surplus
Consumer + producer surplus โ€” where is it maximized?
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๐Ÿƒ Answer
CS (Consumer Surplus) + PS (Producer Surplus) = Total Surplus โ€” maximized at free market equilibrium
Deadweight lossLoss of total surplus from non-equilibrium. Taxes, price controls, monopoly all create DWL.
Tax incidenceMore inelastic side bears more of tax burden. Perfectly inelastic demand: buyers bear 100%.
Price ceiling effectsCeiling below equilibrium: shortage. Total surplus decreases (DWL created). CS may rise or fall.
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Price Controls
Price ceiling = shortage (rent control) ยท Price floor = surplus (minimum wage)
Government price limits distort markets and create inefficiency
Government price limits distort markets and create inefficiency
Price ceilings (max price below equilibrium): create shortages. Rent control, 1970s gas controls. Price floors (min price above equilibrium): create surpluses. Minimum wage, agricultural price supports.
Rent control consequences
Short-run: some renters benefit. Long-run: housing supply shrinks, quality deteriorates, black markets develop.
Minimum wage debate
Simple model: unemployment. Empirical evidence mixed. Monopsony model explains why modest increases may not reduce employment.
Non-price rationing
Queuing, favoritism, rationing, black markets replace price mechanism when controls create shortages.
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๐Ÿƒ Price Controls
Price ceilings vs price floors โ€” what does each cause?
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๐Ÿƒ Answer
Price ceiling = shortage (rent control) ยท Price floor = surplus (minimum wage)
Rent control consequencesShort-run: some renters benefit. Long-run: housing supply shrinks, quality deteriorates, black markets develop.
Minimum wage debateSimple model: unemployment. Empirical evidence mixed. Monopsony model explains why modest increases may not reduce employment.
Non-price rationingQueuing, favoritism, rationing, black markets replace price mechanism when controls create shortages.
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Market Efficiency
Allocative efficiency: P = MC (Marginal Cost) ยท Productive efficiency: minimum ATC (Average Total Cost) โ€” free markets maximize both
Efficient markets maximize total surplus and produce at lowest cost
Efficient markets maximize total surplus and produce at lowest cost
Allocative efficiency: resources go to highest-valued uses (P = MC). Productive efficiency: output at minimum ATC. Perfect competition achieves both long-run. Market failures justify government intervention.
Pareto efficiency
No one can be made better off without making someone else worse off. Competitive equilibrium is Pareto efficient.
Equity vs efficiency
Efficient outcomes may not be fair. Redistributive policies may reduce efficiency while improving equity.
Coase theorem
With clear property rights and low transaction costs, private bargaining can solve externalities without government.
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๐Ÿƒ Market Efficiency
Allocative vs productive efficiency?
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๐Ÿƒ Answer
Allocative efficiency: P = MC (Marginal Cost) ยท Productive efficiency: minimum ATC (Average Total Cost) โ€” free markets maximize both
Pareto efficiencyNo one can be made better off without making someone else worse off. Competitive equilibrium is Pareto efficient.
Equity vs efficiencyEfficient outcomes may not be fair. Redistributive policies may reduce efficiency while improving equity.
Coase theoremWith clear property rights and low transaction costs, private bargaining can solve externalities without government.
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Demand Shifters SPICE
SPICE โ€” Substitute/complement prices, Personal income, Individual tastes, Consumer expectations, # of buyers
Five non-price factors that shift the entire demand curve left or right
Price changes cause MOVEMENT along the curve. Non-price factor changes SHIFT the entire curve.
Substitutes: price of substitute rises, demand for this good rises (right shift). Complements: price rises, demand falls (left shift). Income: normal good โ€” income rises, demand rises. Inferior good โ€” income rises, demand falls. Tastes and preferences: fashion, advertising. Expectations: expect price rise tomorrow, buy more today. Number of buyers: more people, more demand. Only price causes movement along โ€” not a shift.
Substitutes
Pepsi price rises, Coke demand rises โ€” same direction shift
Complements
Gas price rises, SUV demand falls โ€” opposite direction
Expectations
Expect higher future price, buy more now โ€” demand right today
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๐Ÿƒ Demand Shifters SPICE
SPICE
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๐Ÿƒ Answer
SPICE โ€” Substitute/complement prices, Personal income, Individual tastes, Consumer expectations, # of buyers
SubstitutesPepsi price rises, Coke demand rises โ€” same direction shift
ComplementsGas price rises, SUV demand falls โ€” opposite direction
ExpectationsExpect higher future price, buy more now โ€” demand right today
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Supply Shifters ROTTEN
ROTTEN โ€” Resource costs, Other goods prices, Technology, Taxes/subsidies, Expectations, Number of sellers
Six non-price factors that shift the entire supply curve left or right
Cost increases shift supply LEFT. Technology improvements shift supply RIGHT.
Resource costs: input price rise shifts supply left. Technology: improvement lowers costs, shifts right. Taxes: increase costs, shift left. Subsidies: reduce costs, shift right. Expectations: expect higher future price, withhold supply now, shift left. Number of sellers: more sellers, supply right. These SHIFT the curve โ€” price changes only cause movement along it.
Resource costs
Input price rise shifts left. Technology improvement shifts right.
Taxes vs subsidies
Tax shifts left (costs more). Subsidy shifts right (costs less).
Expectations
Expect higher price, hold back supply now โ€” current supply shifts left
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๐Ÿƒ Supply Shifters ROTTEN
ROTTEN
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๐Ÿƒ Answer
ROTTEN โ€” Resource costs, Other goods prices, Technology, Taxes/subsidies, Expectations, Number of sellers
Resource costsInput price rise shifts left. Technology improvement shifts right.
Taxes vs subsidiesTax shifts left (costs more). Subsidy shifts right (costs less).
ExpectationsExpect higher price, hold back supply now โ€” current supply shifts left
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Consumer and Producer Surplus
CS = triangle above price below demand curve. PS = triangle below price above supply curve. Total surplus is maximized at equilibrium.
How competitive markets maximize total surplus and how controls destroy it
Deadweight loss is surplus destroyed when output moves away from equilibrium.
Consumer Surplus (CS): willingness to pay minus price paid. Area below demand curve, above price. Producer Surplus (PS): price received minus minimum acceptable. Area above supply curve, below price. Total Surplus = CS + PS: maximized at competitive equilibrium. Price ceiling creates deadweight loss. Price floor creates deadweight loss. Tax creates two triangles of deadweight loss.
Consumer surplus
Willingness to pay minus actual price โ€” triangle under demand
Producer surplus
Price minus minimum acceptable โ€” triangle above supply
Deadweight loss
Surplus destroyed by controls or taxes โ€” lost to everyone
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๐Ÿƒ Consumer and Producer Surplus
Where are CS and PS on the graph?
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๐Ÿƒ Answer
CS = triangle above price below demand curve. PS = triangle below price above supply curve. Total surplus is maximized at equilibrium.
Consumer surplusWillingness to pay minus actual price โ€” triangle under demand
Producer surplusPrice minus minimum acceptable โ€” triangle above supply
Deadweight lossSurplus destroyed by controls or taxes โ€” lost to everyone
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Tax Incidence
Inelastic side bears the tax. Elastic side escapes it. Who legally pays does not equal who economically bears the burden.
Tax incidence โ€” how tax burden is shared based on elasticity, not legal liability
Perfectly inelastic demand means buyers bear 100 percent. Perfectly elastic demand means sellers bear 100 percent.
Legal incidence: who sends the check to government. Economic incidence: who actually bears the burden through price changes. More inelastic equals less ability to escape the tax. Gasoline (inelastic demand): buyers bear most. Luxury goods (elastic): sellers bear more. Payroll tax: labor bears most burden because labor supply is more inelastic. Tax deadweight loss is larger with more elastic supply and demand.
Inelastic bears more
Cannot adjust quantity โ€” stuck with the tax burden
Elastic escapes
Can switch alternatives easily โ€” passes burden to other side
Legal vs economic
Who pays government does not equal who really bears the burden
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๐Ÿƒ Tax Incidence
Tax incidence โ€” who really bears the tax?
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๐Ÿƒ Answer
Inelastic side bears the tax. Elastic side escapes it. Who legally pays does not equal who economically bears the burden.
Inelastic bears moreCannot adjust quantity โ€” stuck with the tax burden
Elastic escapesCan switch alternatives easily โ€” passes burden to other side
Legal vs economicWho pays government does not equal who really bears the burden
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Price Controls
Floor ABOVE equilibrium creates surplus (minimum wage). Ceiling BELOW equilibrium creates shortage (rent control). Both create deadweight loss.
Price controls and why binding controls always create inefficiency
Non-binding controls have no effect. Only binding controls that prevent equilibrium cause problems.
Price ceiling: legal maximum. Binding if below equilibrium โ€” creates shortage. Examples: rent control, gasoline caps. Effects: shortage, black markets, quality deterioration. Price floor: legal minimum. Binding if above equilibrium โ€” creates surplus. Examples: minimum wage, agricultural supports. Effects: surplus, unemployment in labor market. Both reduce total surplus and create deadweight loss.
Price ceiling
Below equilibrium = binding = shortage. Rent control example.
Price floor
Above equilibrium = binding = surplus. Minimum wage example.
Non-binding
Does not affect market outcome โ€” price never reaches the control
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๐Ÿƒ Price Controls
Price floors vs price ceilings?
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๐Ÿƒ Answer
Floor ABOVE equilibrium creates surplus (minimum wage). Ceiling BELOW equilibrium creates shortage (rent control). Both create deadweight loss.
Price ceilingBelow equilibrium = binding = shortage. Rent control example.
Price floorAbove equilibrium = binding = surplus. Minimum wage example.
Non-bindingDoes not affect market outcome โ€” price never reaches the control
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Total Revenue Test
Elastic: price up means total revenue DOWN. Inelastic: price up means total revenue UP. Unit elastic: no change in revenue.
Using elasticity to predict how price changes affect total revenue
Drug addiction and necessities are inelastic โ€” price hikes raise revenue. Luxuries are elastic โ€” price hikes reduce revenue.
Total Revenue = Price times Quantity. Elastic (PED greater than 1): quantity drops more than price rises, so TR falls when price rises. Inelastic (PED less than 1): quantity drops less than price rises, so TR rises when price rises. Unit elastic: TR unchanged. OPEC: inelastic oil demand means restricting supply boosts revenue. Agriculture: inelastic food demand means bumper crop lowers farmer revenue. Drug war: inelastic demand means supply reduction raises dealer revenue.
Elastic + price up
Total revenue falls โ€” quantity drop offsets price gain
Inelastic + price up
Total revenue rises โ€” quantity drop is small
OPEC application
Inelastic oil demand means restricting supply raises cartel revenue
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๐Ÿƒ Total Revenue Test
Elasticity and total revenue โ€” what happens when price rises?
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๐Ÿƒ Answer
Elastic: price up means total revenue DOWN. Inelastic: price up means total revenue UP. Unit elastic: no change in revenue.
Elastic + price upTotal revenue falls โ€” quantity drop offsets price gain
Inelastic + price upTotal revenue rises โ€” quantity drop is small
OPEC applicationInelastic oil demand means restricting supply raises cartel revenue
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