Demand is the relationship between a good's price and the quantity buyers are willing and able to purchase, other things constant. The law of demand: price up → quantity demanded down (and vice versa). Two forces drive it: - Substitution effect: when a good's price rises, buyers switch toward relatively cheaper substitutes — beef gets expensive, the grocery cart fills with chicken. - Income effect: a higher price shrinks the purchasing power of a given paycheck, so buyers afford less.
[GRAPH: Demand curve. X-axis: "Quantity", Y-axis: "Price". Downward-sloping line labeled D. Arrow along the curve from (Q=10, P=$8) to (Q=20, P=$5) labeled "change in quantity demanded (movement along)". A second, rightward-shifted parallel line labeled D₂ with arrow labeled "change in demand (shift)".]
The distinction the exam tests relentlessly: - A change in the good's own price → movement along the curve = change in quantity demanded. - A change in anything else → the whole curve shifts = change in demand.
Determinants of demand (shifters):
| Shifter | Rightward (increase) example |
|---|---|
| Tastes / preferences | A safety report improves a product's reputation |
| Income — normal goods | Household incomes rise |
| Income — inferior goods | Household incomes fall (instant noodles, bus rides, secondhand goods) |
| Price of a substitute | Substitute's price rises (transit fares ↑ → demand for gasoline ↑) |
| Price of a complement | Complement's price falls (cheaper gas → demand for SUVs ↑) |
| Buyer expectations | Buyers expect higher prices later → buy more now |
| Number of buyers | A city's population grows (more renters → rental demand ↑) |
Supply is the relationship between price and the quantity sellers are willing and able to produce. The law of supply: price up → quantity supplied up, because higher prices cover rising marginal costs and draw in production.
Determinants of supply (shifters):
| Shifter | Rightward (increase) example |
|---|---|
| Input / resource prices | Wholesale ingredient or wage costs fall |
| Technology | Better production methods |
| Taxes / subsidies | Per-unit tax cut or new subsidy |
| Seller expectations | Sellers expect lower future prices → sell more now |
| Number of sellers | New firms enter the market |
| Prices of related outputs | An alternative product the firm could make becomes less profitable |
Same grammar as demand: own price → movement along (change in quantity supplied); anything else → shift (change in supply).
Where the curves cross, quantity demanded = quantity supplied: the market clears at equilibrium price P* and quantity Q*.
[GRAPH: Market equilibrium. X-axis "Quantity", Y-axis "Price". Downward D and upward S crossing at (Q = 100, P = $6), dashed lines to both axes. At P = $9 a horizontal gap between the curves labeled "Surplus (Qs > Qd)". At P = $3 a gap labeled "Shortage (Qd > Qs)".]
The same logic runs labor markets. If a small business posts a wage and gets far more qualified applicants than openings, the offered wage is above the market-clearing level — a surplus of labor. Market pressure pushes the wage down until applicants and openings match.
Solving equilibrium algebraically: given Qd = 90 − 3P and Qs = 10 + 5P, set them equal: 90 − 3P = 10 + 5P → 80 = 8P → P* = 10, and Q* = 90 − 3(10) = 60. Always check by plugging into the other equation: Qs = 10 + 50 = 60 ✓.
| Shift | P* | Q* |
|---|---|---|
| Demand ↑ (right) | ↑ | ↑ |
| Demand ↓ (left) | ↓ | ↓ |
| Supply ↑ (right) | ↓ | ↑ |
| Supply ↓ (left) | ↑ | ↓ |
Signature patterns: after a demand shift, P and Q move the same direction; after a supply shift, they move in opposite directions.
When both curves shift, one of P or Q is determinate and the other depends on the relative sizes of the shifts — the correct answer is "indeterminate" (or "cannot be determined").
| Case | P* | Q* |
|---|---|---|
| D↑ and S↑ | indeterminate | ↑ |
| D↓ and S↓ | indeterminate | ↓ |
| D↑ and S↓ | ↑ | indeterminate |
| D↓ and S↑ | ↓ | indeterminate |
Shortcut: write each shift's pressure on P and Q separately. Where the two shifts agree, that variable is determined; where they conflict, it is indeterminate. Never guess a direction for the indeterminate variable.
The official CLEP exam uses a question format in which each answer choice is a pair of directions, presented as a two-column table:
| Price of Oranges | Quantity of Oranges | |
|---|---|---|
| A) | Increase | Increase |
| B) | Decrease | Decrease |
| C) | Decrease | Increase |
| D) | Increase | Decrease |
| E) | No change | Decrease |
Attack these mechanically: identify which curve shifts and which way, read P and Q off the four-single-shifts table, and match the pair. Question 10 below practices this format.
Q1 — A. Only a change in the good's own price moves buyers along a fixed demand curve (a change in quantity demanded). B is an income shifter; C is a substitute's price changing, which shifts gasoline demand; D is a tastes shifter; E changes the number of buyers — all four move the whole curve. Fix rule: own price → movement along; anything else on the shifter list → shift.
Q2 — A. Switching from pricier beef to relatively cheaper chicken is the substitution effect — one of the two reasons demand curves slope downward. B misreads the mechanism: the income effect is about the purchasing power of an unchanged paycheck, not falling wages. C is backwards — buying less beef at a higher beef price is a movement along, and toward less, not a rightward shift. D names the wrong law; this is buyer behavior. E is the opposite of what happened: buying less as price rises confirms the law of demand. Fix rule: substitution effect = switching to the cheaper alternative; income effect = your dollars stretch less — both explain downward-sloping demand.
Q3 — D. For an inferior good, falling income shifts demand right: people trade down to noodles, so both price and quantity rise. A applies the normal-good rule to an inferior good — the classic reversal error. B mislabels a shift as a movement along; income is a shifter, not the good's own price. C moves the wrong curve — nothing changed on the seller side. E ignores demand-side changes entirely; markets move even when costs don't. Fix rule: inferior goods flip the income rule — income ↓ → demand ↑ (and vice versa).
Q4 — C. More applicants than openings means quantity supplied of labor exceeds quantity demanded — the wage is above equilibrium (a surplus). The wage falls; as it falls, fewer people apply (Qs ↓) and employers want more workers (Qd ↑) until the market clears. A moves the wage the wrong way — raising it would worsen the surplus. B and D expect curves to shift to fix a disequilibrium, but no determinant changed; price (the wage) does the adjusting. E confuses surplus with shortage: excess applicants is excess supply, not excess demand. Fix rule: too many sellers at the current price = surplus = price falls; the curves stay put.
Q5 — B. A complement's price rose, so demand for SUVs shifts left: price and quantity of SUVs both fall. A and E move the supply curve, but nothing changed automakers' costs. C treats the complement like a substitute — the direction is reversed. D calls it a movement along, but the price of gasoline changed, not the price of SUVs; cross-price effects arrive as demand shifts. Fix rule: complement's price ↑ → demand for the paired good ↓; only the good's OWN price causes movement along.
Q6 — E. Costlier ingredients raise sellers' per-unit costs, shifting restaurant-meal supply left. A is the good's own price — a movement along the supply curve, never a shift. B adds sellers, shifting supply right. C lowers costs — also a rightward shift. D improves technology — rightward again. Fix rule: ask "does this change sellers' costs or numbers?" Costs up or sellers out = supply left; the good's own price never shifts its own curve.
Q7 — E. Set Qd = Qs: 90 − 3P = 10 + 5P → 80 = 8P → P = 10; Q = 90 − 30 = 60. Check: Qs = 10 + 50 = 60 ✓. A comes from dividing 80 by 10 (mixing up coefficients). B results from adding the constants (90 + 10 = 100, then 100/8) instead of subtracting. C comes from subtracting the coefficients (5 − 3 = 2) instead of adding them when collecting P terms. D uses 80/4, a coefficient slip. Fix rule: set Qd = Qs, collect P terms by ADDING the coefficients on opposite sides, then verify Q in both equations.
Q8 — B. More renters → demand right (pressure: P↑, Q↑). Higher landlord costs → supply left (pressure: P↑, Q↓). Both shifts push rent up, so price rises; they push quantity in opposite directions, so quantity is indeterminate. A guesses a direction for the conflicted variable. C and D reverse which variable is determined. E over-applies "indeterminate" — the two shifts agree on price. Fix rule: in double shifts, the variable both shifts agree on is determined; the one they fight over is indeterminate.
Q9 — C. D↑ pushes P↑, Q↑; S↑ pushes P↓, Q↑. Quantity rises on both counts; price is pushed both ways, so it is indeterminate. A guesses the indeterminate price upward. B guesses it downward and wrongly makes quantity uncertain. D swaps which variable is known. E ignores that both shifts agree on quantity. Fix rule: D and S both increase → Q definitely rises, P depends on shift sizes — write "indeterminate."
A severe freeze destroys a large share of the orange crop. Which of the following shows the effect on the equilibrium price and quantity of oranges?
| Price of Oranges | Quantity of Oranges | |
|---|---|---|
Q10 — D. The freeze destroys productive capacity: supply shifts left, so price rises and quantity falls — the supply-shift signature of opposite movements. A is the demand-increase pattern; nothing changed buyers' willingness. B is the demand-decrease pattern. C is the supply-increase pattern, the exact reverse of a crop loss. E freezes the price, but with fewer oranges at every price, buyers bid the price up. Fix rule: supply shifts move P and Q in OPPOSITE directions; demand shifts move them together — match the pair accordingly.
Q11 — D. Expecting lower prices later, buyers postpone purchases: demand shifts left today, so today's price and quantity both fall. A reverses the expectation logic — rushing to buy fits expected price increases. B describes sellers' behavior under expected price increases (withholding to sell high later); here sellers would if anything sell sooner. C mislabels a shift: expectations are a demand shifter, and the good's own current price hasn't changed on its own. E ignores that expectations move markets immediately — that is the whole point of the expectations shifter. Fix rule: expected future price ↓ → demand ↓ today; expected future price ↑ → demand ↑ today.
Q12 — C. A price increase alone doesn't reveal which curve moved: higher demand raises P and Q, but reduced supply raises P while lowering Q. Without quantity data, the owner's inference is premature. A and B commit the "price rose so demand rose" fallacy — B even garbles the law of demand, which relates price to quantity demanded along a curve, not to shifts. D is backwards: shifters change demand without any prior change in the good's own price. E is false — demand shifts are precisely one of the things competitive prices respond to. Fix rule: to diagnose which curve shifted, check quantity: P↑ with Q↑ = demand rose; P↑ with Q↓ = supply fell.
Q1 — A. Only a change in the good's own price moves buyers along a fixed demand curve (a change in quantity demanded). B is an income shifter; C is a substitute's price changing, which shifts gasoline demand; D is a tastes shifter; E changes the number of buyers — all four move the whole curve. Fix rule: own price → movement along; anything else on the shifter list → shift.
Q2 — A. Switching from pricier beef to relatively cheaper chicken is the substitution effect — one of the two reasons demand curves slope downward. B misreads the mechanism: the income effect is about the purchasing power of an unchanged paycheck, not falling wages. C is backwards — buying less beef at a higher beef price is a movement along, and toward less, not a rightward shift. D names the wrong law; this is buyer behavior. E is the opposite of what happened: buying less as price rises confirms the law of demand. Fix rule: substitution effect = switching to the cheaper alternative; income effect = your dollars stretch less — both explain downward-sloping demand.
Q3 — D. For an inferior good, falling income shifts demand right: people trade down to noodles, so both price and quantity rise. A applies the normal-good rule to an inferior good — the classic reversal error. B mislabels a shift as a movement along; income is a shifter, not the good's own price. C moves the wrong curve — nothing changed on the seller side. E ignores demand-side changes entirely; markets move even when costs don't. Fix rule: inferior goods flip the income rule — income ↓ → demand ↑ (and vice versa).
Q4 — C. More applicants than openings means quantity supplied of labor exceeds quantity demanded — the wage is above equilibrium (a surplus). The wage falls; as it falls, fewer people apply (Qs ↓) and employers want more workers (Qd ↑) until the market clears. A moves the wage the wrong way — raising it would worsen the surplus. B and D expect curves to shift to fix a disequilibrium, but no determinant changed; price (the wage) does the adjusting. E confuses surplus with shortage: excess applicants is excess supply, not excess demand. Fix rule: too many sellers at the current price = surplus = price falls; the curves stay put.
Q5 — B. A complement's price rose, so demand for SUVs shifts left: price and quantity of SUVs both fall. A and E move the supply curve, but nothing changed automakers' costs. C treats the complement like a substitute — the direction is reversed. D calls it a movement along, but the price of gasoline changed, not the price of SUVs; cross-price effects arrive as demand shifts. Fix rule: complement's price ↑ → demand for the paired good ↓; only the good's OWN price causes movement along.
Q6 — E. Costlier ingredients raise sellers' per-unit costs, shifting restaurant-meal supply left. A is the good's own price — a movement along the supply curve, never a shift. B adds sellers, shifting supply right. C lowers costs — also a rightward shift. D improves technology — rightward again. Fix rule: ask "does this change sellers' costs or numbers?" Costs up or sellers out = supply left; the good's own price never shifts its own curve.
Q7 — E. Set Qd = Qs: 90 − 3P = 10 + 5P → 80 = 8P → P = 10; Q = 90 − 30 = 60. Check: Qs = 10 + 50 = 60 ✓. A comes from dividing 80 by 10 (mixing up coefficients). B results from adding the constants (90 + 10 = 100, then 100/8) instead of subtracting. C comes from subtracting the coefficients (5 − 3 = 2) instead of adding them when collecting P terms. D uses 80/4, a coefficient slip. Fix rule: set Qd = Qs, collect P terms by ADDING the coefficients on opposite sides, then verify Q in both equations.
Q8 — B. More renters → demand right (pressure: P↑, Q↑). Higher landlord costs → supply left (pressure: P↑, Q↓). Both shifts push rent up, so price rises; they push quantity in opposite directions, so quantity is indeterminate. A guesses a direction for the conflicted variable. C and D reverse which variable is determined. E over-applies "indeterminate" — the two shifts agree on price. Fix rule: in double shifts, the variable both shifts agree on is determined; the one they fight over is indeterminate.
Q9 — C. D↑ pushes P↑, Q↑; S↑ pushes P↓, Q↑. Quantity rises on both counts; price is pushed both ways, so it is indeterminate. A guesses the indeterminate price upward. B guesses it downward and wrongly makes quantity uncertain. D swaps which variable is known. E ignores that both shifts agree on quantity. Fix rule: D and S both increase → Q definitely rises, P depends on shift sizes — write "indeterminate."
Q10 — D. The freeze destroys productive capacity: supply shifts left, so price rises and quantity falls — the supply-shift signature of opposite movements. A is the demand-increase pattern; nothing changed buyers' willingness. B is the demand-decrease pattern. C is the supply-increase pattern, the exact reverse of a crop loss. E freezes the price, but with fewer oranges at every price, buyers bid the price up. Fix rule: supply shifts move P and Q in OPPOSITE directions; demand shifts move them together — match the pair accordingly.
Q11 — D. Expecting lower prices later, buyers postpone purchases: demand shifts left today, so today's price and quantity both fall. A reverses the expectation logic — rushing to buy fits expected price increases. B describes sellers' behavior under expected price increases (withholding to sell high later); here sellers would if anything sell sooner. C mislabels a shift: expectations are a demand shifter, and the good's own current price hasn't changed on its own. E ignores that expectations move markets immediately — that is the whole point of the expectations shifter. Fix rule: expected future price ↓ → demand ↓ today; expected future price ↑ → demand ↑ today.
Q12 — C. A price increase alone doesn't reveal which curve moved: higher demand raises P and Q, but reduced supply raises P while lowering Q. Without quantity data, the owner's inference is premature. A and B commit the "price rose so demand rose" fallacy — B even garbles the law of demand, which relates price to quantity demanded along a curve, not to shifts. D is backwards: shifters change demand without any prior change in the good's own price. E is false — demand shifts are precisely one of the things competitive prices respond to. Fix rule: to diagnose which curve shifted, check quantity: P↑ with Q↑ = demand rose; P↑ with Q↓ = supply fell.