IB Economics: Price Elasticity of Supply (PES) FAQ

You need the formula (%ΔQs ÷ %ΔP), the five determinants — time, spare capacity, factor mobility, stock levels, and production period — and how to read a PES value against 1 to call supply elastic, inelastic, or unit elastic. Examiners expect you to link this to real-world supply diagrams.

PES = percentage change in quantity supplied ÷ percentage change in price. A value above 1 means elastic supply, below 1 means inelastic, and exactly 1 means unit elastic. Always show your working — examiners award method marks even if your final number is slightly off.

Five factors: how much time producers have to respond, spare production capacity, how easily factors of production can move between uses, the level of stocks or inventories held, and the length of the production period. More time and more spare capacity both push supply toward elastic.

Elastic supply (PES > 1) means quantity supplied changes proportionally more than price — think of manufactured goods with spare factory capacity. Inelastic supply (PES < 1) means quantity barely moves even with a big price change — classic examples are fresh produce or housing in the short run.

Supply gets more elastic the longer producers have to adjust. In the momentary period supply is fixed (perfectly inelastic), in the short run firms can vary some inputs, and in the long run they can build new capacity, making supply far more responsive to price. Examiners love this as an essay theme.

Draw a standard supply curve on price-quantity axes, then show elasticity through the curve's steepness: a steep, near-vertical curve represents inelastic supply, while a flatter curve shows elastic supply. Label both axes correctly, and if comparing time periods, draw three separate supply curves rotating outward from the same origin.

The biggest one I mark down every year: confusing a shift in the supply curve (caused by cost or technology changes) with a movement along it (caused by price alone). Others include forgetting to state whether PES is elastic or inelastic after calculating it, and mixing up the PES and PED formulas under time pressure.

PES rarely stands alone as a full essay; it typically appears as part (a) of a Paper 1 question worth around 10 marks, or as a calculation and short-answer sub-question in Paper 2 data response worth 2-4 marks. HL Paper 3 can also test it through quantitative extended-response questions.

PED measures how responsive consumers are to a price change; PES measures how responsive producers are. Both use the same percentage-change structure, but PED is almost always negative (law of demand) while PES is almost always positive since price and quantity supplied move in the same direction.

In almost every normal case, no — PES is positive because higher prices give producers an incentive to supply more, so price and quantity supplied move together. A negative PES would be unusual and generally isn't tested in the IB Economics guide, so don't spend exam time hunting for exceptions.

Yes, the core PES concept, formula and diagrams are identical for SL and HL — both are examined in Paper 1 and Paper 2. HL students additionally meet PES in Paper 3's quantitative questions, where you might calculate PES from a data table and then evaluate a policy implication.

The most useful thing you can do is check your child can state the formula from memory and explain, in plain English, why time makes supply more elastic — that's the concept examiners return to most. Past-paper practice under timed conditions matters more here than buying extra textbooks.