Supply = quantity offered at each price
Supply is the quantity of a good or service that producers are willing and able to offer for sale at each price in a given period. Both willingness and ability matter — a firm might want to sell more but be unable to without higher prices to cover rising costs.
Supply curve slopes upward
Other things equal, a higher price makes production more profitable and encourages firms to supply more, so the supply curve slopes upward from left to right. A change in the price of the good itself causes a movement along the curve: an extension (rise in price, more supplied) or a contraction (fall in price, less supplied).
Market supply sums individual supplies
At each price, add up the quantity every producer is willing to supply. Market supply is found by aggregating (summing horizontally) the individual supply curves — e.g. at one price, if firm A supplies 30 units and firm B supplies 50, market supply is 80.
Drawn from real examiner reports.
Shift vs movement along supply
A change in the price of the good itself moves you along the supply curve (extension/contraction). A change in a condition of supply — costs of production, technology, taxes, subsidies, weather — shifts the whole curve. Mixing these up is one of the most penalised errors.
In June 2023 Paper 11 (Multiple Choice) Q9, only 39% answered correctly: a rise in farm workers' wages raises costs and *shifts* the supply curve, whereas a rise in consumers' incomes raises demand and causes a *movement along* the supply curve.
Define elasticity in percentage terms
When a question is about responsiveness, vague phrasing loses marks. Saying a "small change in supply" is not enough — the precise idea is that a given percentage change in price causes a smaller percentage change in quantity supplied (inelastic supply). Some candidates even define inelastic demand by mistake.
November 2022 Paper 22 (Structured) Q4(a): the report noted answers lacked precision, with candidates writing about a "small change in supply" rather than percentage changes, and a few defining inelastic demand instead.
State the shift direction and why
Naming an influence (e.g. a fall in input costs, a new subsidy, better technology) earns little credit on its own. You must state the direction: does supply increase (curve shifts right) or decrease (curve shifts left)? Then say why — costs of production have fallen so firms can supply more at every price.
June 2023 Paper 21 (Structured) Q1(d): candidates identified valid influences on the supply of coal (storage, building a new mine) but did not state whether these made supply more or less responsive, so they scored poorly.
Tax shifts left, subsidy shifts right
An indirect tax raises firms' costs, so supply falls and the curve shifts left. A subsidy lowers costs, so supply rises and the curve shifts right. Students often reverse these. Anchor it to costs: higher costs -> less supplied at every price -> leftward shift; lower costs -> rightward shift.
Supply = willing AND able
Supply means producers are both willing and able to offer a good for sale at a given price — not just what they would like to sell. A firm may want to supply more but be unable to unless a higher price covers the extra cost. Include both willingness and ability in the definition.
8-mark discuss: two sides + judgement
Part (d) rewards analysis (AO2) and evaluation (AO3), not lists. Develop each point as a chain (cause -> effect), argue both sides, then give a justified judgement. Weak answers state unexplained points or run short after over-writing the short parts.
State direction, then explain via costs
For any supply shift, state the direction (increase = right, decrease = left) and explain via costs — a condition of supply changes firms' costs, so they supply more or less at every price. Naming the cause without the direction earns little.
Match the command word
Answer to the command word: define (2) = precise meaning; explain (4) = a developed reason; analyse (6) = a chain of reasoning; discuss (8) = both sides plus a judgement. Answering below the command word's demand caps the mark.
Supply is the quantity of a good or service that producers are willing and able to offer for sale at each price, over a given period of time. Both parts matter: a firm may want to sell more but be unable to unless a higher price covers the extra cost.
Other things equal, a higher price makes selling more profitable, so producers supply more. The supply curve slopes upward from bottom-left to top-right.
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