IB Economics: Supply & the Law of Supply — FAQs

Supply is the quantity of a good a firm is willing and able to sell at each price over a given period. The law of supply states that, other factors constant, price and quantity supplied are positively related. It's examined via diagram-drawing, data-response questions and the HL paper's quantitative elasticity questions.

The supply curve slopes upward because higher prices make production more profitable, so firms are willing to supply more and new firms enter the market. Existing firms also expand output by hiring extra workers or running machinery for longer, even though marginal cost rises as they do.

A movement along the supply curve happens only when price changes — you move to a new point on the same curve. A shift of the whole curve happens when a non-price determinant changes, like costs, technology or the number of sellers. Mixing these up is the single most common error I see in Paper 1 responses.

The main non-price determinants are: costs of production (wages, raw materials, rent), technology, government intervention (taxes and subsidies), the number of firms in the market, prices of related goods in production (joint or competitive supply), and producer expectations of future prices.

An indirect tax (specific or ad valorem) raises a firm's effective cost per unit, shifting supply left/upward — less is supplied at every price. A subsidy does the opposite: it lowers effective cost, shifting supply right/downward, so more is supplied at every price. Both change the market equilibrium and the tax/subsidy incidence between consumers and producers.

Price elasticity of supply measures how responsive quantity supplied is to a change in price, calculated as %ΔQs ÷ %ΔP. PES greater than 1 is elastic supply (firms can ramp up output easily); PES less than 1 is inelastic (output is hard to expand quickly, common with agricultural goods).

Joint supply is when producing one good automatically produces another as a by-product — beef and leather, for example — so a rise in demand for one increases supply of both. Competitive supply is when a firm chooses between producing one good or another using the same resources, like a farmer choosing wheat over barley.

Draw the diagram first, then write around it — examiners mark the diagram and text together, but a correct diagram anchors your explanation. Label both axes, both curves before and after the shift, and the new equilibrium clearly. Then explain the causal chain in full sentences, not just 'supply shifts left'.

SL and HL cover the same core supply theory — the law of supply, determinants, shifts versus movements. HL adds quantitative work: calculating price elasticity of supply from data, and the extra HL-only Paper 3 which tests numerical and diagrammatic supply problems in more depth than SL ever requires.

The three recurring mistakes I mark every year: confusing a shift with a movement, forgetting to relabel the curve (S1, S2) after a shift, and stating 'supply increases' when a price rise actually causes a movement along the existing curve, not a genuine increase in supply.

Supply and demand together form Microeconomics, the largest of the IB Economics guide's four syllabus themes by teaching time. Expect at least one Paper 1 essay question, several Paper 2 data-response questions across the course, and — at HL — a guaranteed quantitative supply or elasticity question on Paper 3.

Yes — the core supply and demand theory in IB Economics and A-Level Economics is essentially the same neoclassical microeconomics, and a student moving between the two systems won't need to relearn the fundamentals. The main difference is IB's heavier emphasis on real-world case studies and its structured internal assessment commentary.

Look for resources built around the current IB Economics guide's command terms and syllabus themes, not generic economics textbooks. On revisionprep.com you'll find Economics Revision Notes covering supply-side theory, Topical Worksheets for diagram practice, and Mock Papers that mirror real Paper 1/2/3 structure and mark schemes.

It's usually a presentation issue, not an understanding gap — missing axis labels, unlabelled curve shifts, or skipping the explicit 'new equilibrium' statement examiners look for. This is a mark-scheme habit, and it's fixable with focused diagram practice rather than more theory revision.