Business Economics Part 2: Elasticity — The Measure of Stubbornness

Why does a price cut sometimes earn a seller less?

The degree of responsiveness of quantity demanded to a change in price — the working definition every examiner starts from.

In one line: Business Economics Part 2 — elasticity, the measure of stubbornness, in one read.

Related: Business Economics Part 1: Demand and Supply — The Equilibrium Dance

✪ Key points — the 30-second version

• Elasticity is a ratio of percentages, not of rupees: PED = %ΔQ ÷ %ΔP
• |PED| > 1 elastic · < 1 inelastic · = 1 unitary · 0 perfectly inelastic ·

∞ perfectly elastic

• Total outlay test: price ↑ and spending ↑ → inelastic; price ↑ and

spending ↓ → elastic; spending flat → unitary

• YED signs: normal (+), luxury (> 1), necessity (0–1), inferior (−)
• XED signs: substitutes (+), complements (−), unrelated (0)
• PES grows with time: momentary < short < long run
• Tax burden falls mainly on the less elastic side — the logic behind

India’s 40% GST slab on sin goods

• Giffen ≠ inferior: every Giffen good is inferior, almost no inferior good

is Giffen

Part 1 gave demand and supply their curves; this part gives them a speedometer. Same market, new question — not which way, but how much.

1. Elasticity: The Measure of Stubbornness

Part 1 ended with the law of demand: price up, quantity down. But buyers are not equally obedient. Cut the price of salt by half and the kitchen barely notices. Cut the price of a flagship phone by half and queues form. The law of demand gives the direction; elasticity gives the magnitude. And magnitude is where marks, pricing decisions and tax policy all live.

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One definition to memorise: elasticity of demand is the degree of responsiveness of quantity demanded to a change in price, measured as the ratio of the percentage change in quantity to the percentage change in price. Percentages, not units. That is what lets us compare salt with phones, and India with Indonesia, on one scale.

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2. PED: The Formula and Its Five Degrees

PED = percentage change in quantity demanded ÷ percentage change in price. By the law of demand, PED is negative; exams usually report its absolute value. Five degrees, five pictures:

The trap inside the table: flatness is not elasticity. Slope measures absolute change (ΔQ/ΔP); elasticity measures percentage change. Along a straight-line demand curve, slope is constant but elasticity is not —

bottom. The top half is elastic, the bottom half inelastic, the midpoint unitary. Diagram 1 should show exactly this.

3. Two Methods, Two Answers — Percentage vs Midpoint

The percentage (proportionate) method divides each change by its original value: PED = (ΔQ/Q₀) ÷ (ΔP/P₀). Simple. But asymmetric: going ₹10 → ₹12 is a 20% rise, yet ₹12 → ₹10 is only a 16.7% fall. Same move, different elasticity depending on direction.

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The midpoint (arc) method kills the asymmetry by dividing by the average of the two ends:

PED = [ΔQ ÷ ((Q₁+Q₂)/2)] ÷ [ΔP ÷ ((P₁+P₂)/2)]

Use it whenever a question gives two points on the curve; use the simple method when only the initial base matters or the question dictates it. Numerical 1 below works the midpoint fully. Exams award steps for the method, not just the answer.

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4. The Total Outlay Test — the Examiner’s Favourite Shortcut

Total outlay (expenditure) = price × quantity. Since price and quantity move opposite ways, the product’s direction reveals who wins:

The test works in reverse for a price cut. It needs no formulas — only the sign of the change. Which is why objective papers love it, and why Numerical 2 below solves in three lines. Two limits to state in an answer: it cannot measure the exact coefficient, and it does not apply to supply (or income changes).

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5. The Family: PES, YED, XED

Income elasticity (YED) = %ΔQ ÷ %Δ income. The sign is the story: positive for normal goods, and within them greater than 1 for luxuries (income grows, spending grows faster) but between 0 and 1 for necessities (spending grows slower). Negative YED marks inferior goods. Coarse grain, generic brands, second-hand scooters in a booming economy. India’s growth story is a YED story: as incomes rise, demand for two-wheelers premiumises and demand for hostel-style budget travel lags.

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Cross elasticity (XED) = %ΔQ of X ÷ %ΔP of Y. Positive → substitutes (tea and coffee); negative → complements (cars and petrol). Near zero → unrelated. XED is how economists define “the same market”: high positive XED between two products means one firm’s price is the other firm’s business.

Price elasticity of supply (PES) answers the same how-much question for sellers. Its master determinant is time: in the momentary period supply is fixed (PES ≈ 0. Think vegetables arriving at dawn’s mandi), the short run allows more labour and shifts, and the long run allows capacity itself to change. Add storage (storable onions versus perishable tomatoes) and factor mobility, and you have every PES determinant an exam can ask.

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6. Determinants and Indian Prices

Why is one demand stubborn and another sensitive? Four determinants do most of the work, each with a live Indian example (prices as tracked in early September 2026):

• Substitutes. Salt has none — demand close to perfectly inelastic, which

is why its price never makes news. Petrol in a city of commuters is nearly so (Delhi pumps held near ₹102 a litre through early September), yet slightly elastic over years as metro lines, EVs and work-from-home accumulate substitutes patience can build.

• Proportion of income. A ₹30 rise in salt goes unnoticed; the same rise

on a ₹942 domestic LPG cylinder in Delhi bites enough to make headlines — and policy answers with a ₹300-per-cylinder DBT under PM Ujjwala, in effect cutting the price for the most sensitive households.

• Necessity vs luxury. Cooking fuel is a necessity (low YED, low PED);

the flagship phone is a luxury (YED well above 1) — which is why phone demand surges in bonus season and festival sales, and cylinder demand doesn’t.

• Time. Onion demand is inelastic in any given week; over a season,

households substitute and kitchen gardens respond — elasticity grows as the horizon lengthens.

7. Tax Incidence: Why the Inelastic Side Pays

When government taxes a good, who actually pays? Not whoever writes the cheque. The less elastic side of the market does. Inelastic demand means buyers cannot easily leave, so sellers pass the tax on. Inelastic supply means sellers cannot easily leave, so they absorb it.

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This is the economics behind “sin taxes”. India’s GST 2.0 reform (rates rationalised to 5% and 18% from 22 September 2025) created a special 40% demerit slab for sin and luxury goods, with tobacco and pan masala notified at 40% effective 1 February 2026. Demand for these goods is habit-driven and price-inelastic, so consumption barely falls while revenue stays fat. Exactly the combination a finance ministry wants from a small base, and exactly the sentence a two-mark exam answer needs.

The counterpoint for a five-marker: if the policy goal is health rather than revenue, inelastic demand defeats it. Quantity responds little, and the burden lands regressively on addicted, often poorer, consumers.

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8. How Exams Probe This Topic

• CA Foundation / CUET / BBA entrances: compute PED by both methods,

classify goods by YED and XED signs, apply the total outlay test.

• UGC NET Commerce: determinants of elasticity, incidence of taxation,

elasticity along a linear demand curve, cross-elasticity and market definition.

• UPSC Economy (Prelims + Mains): why indirect taxes lean on inelastic

goods; subsidy and price-control questions dressed as current affairs (LPG DBT, onion export policy).

• The favourite trick in all of them: hand you two price–quantity points and

ask for elasticity by name — say “midpoint method” in your method line before computing, or lose the step.

PED = %ΔQ ÷ %ΔP    |PED| > 1 elastic, < 1 inelastic
outlay test: price ↑ spend ↑ = inelastic; spend flat = unitary
YED + normal, − inferior; XED + substitutes, − complements

9. Quick Revision: One-Glance Facts

• PED = %ΔQ ÷ %ΔP · signs: YED normal/inferior · XED substitutes/complements
• Elastic top, inelastic bottom, unitary midpoint of a straight-line demand

curve

• Outlay test: P↑ TE↑ inelastic · P↑ TE↓ elastic · TE flat unitary
• YED: luxury > 1 > necessity > 0 > inferior
• PES order: momentary < short run < long run
• Tax burden → less elastic side · sin goods taxed because demand is

inelastic

• Giffen: inferior + sizeable income effect overwhelming substitution effect

Practice Corner: Four Solved Numericals (with Answers)

✎ 1 — PED by the midpoint method. A shop raises price from ₹20 to ₹24. Quantity falls from 100 to 80 units. %ΔQ = −20 ÷ 90 = −0.222; %ΔP = 4 ÷ 22 = 0.182. PED = −0.222/0.182 ≈ −1.22 → elastic. (Note: the simple method would give 20/100 ÷ 4/20 = 1.0. The two methods disagree, and Numerical answers must name theirs.)

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✎ 2 — The outlay test. Price rises ₹10 → ₹12; quantity falls 100 → 90. TE: ₹1,000 → ₹1,080. Spending rose with price → inelastic demand (confirm: simple PED = 10% ÷ 20% = 0.5).

✎ 3 — YED classification. Income rises ₹50,000 → ₹60,000 (+20%); cinema trips rise 4 → 6 (+50%). YED = 50 ÷ 20 = +2.5 → a normal good and a luxury.

✎ 4 — XED sign. Tea’s price rises 10%; a household’s coffee purchases rise 5%. XED = 5 ÷ 10 = +0.5 → substitutes (weak ones — coffee responds only half as fast as tea’s price moves).

The Case Lens: The Onion Story

Onion is India’s running lesson in inelastic demand meeting unstable supply. Households cannot easily substitute it and cannot store it, so when the monsoon damages the crop, the demand curve’s steepness turns a supply shock into a price spike. And voters’ tears into policy. The record: exports were banned in December 2023, the ban was lifted in May 2024 with a minimum export price of $550 a tonne, the MEP went in September 2024, a 20% export duty was scrapped from 1 April 2025, and by 2026 exports are free.

Each restriction is supply management aimed at the domestic price of a good whose buyers barely respond to price. The exact opposite of the sin-tax logic, and the same elasticity concept running in reverse. In an answer, name the concept (inelastic demand + supply shock), give the direction (price spike), and cite one dated policy move — that trio is a full case answer.

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The Three Classic Traps (Where Beginners Slip)

• Giffen ≠ inferior. Every Giffen good is inferior; almost no inferior

good is Giffen. Giffen needs the income effect to overwhelm the substitution effect. A famous 2008 study found it in rice for very poor households in China, which is how rare it is. Veblen goods (snob appeal) are a different exception altogether: demand rises with price because the price is the point.

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• Midpoint vs simple method. Same data, different PED (see Numerical 1:

1.22 vs 1.0). Questions that give two points want the midpoint; write the method name down.

• Flat = elastic? Only relatively. Slope is absolute, elasticity is

percentage. A straight-line demand curve carries every elasticity from ∞ to 0 along its length — never call it “an elastic curve” wholesale.

🧠 Memory tricks — the 20-second revision

🧠 Chant: “Percentage over percentage — Q on top, P below.” 🧠 Outlay: “Price up, purse up — stubborn buyer.” 🧠 Signs: “YED — rich buy more (+). XED. Rivals rise together (+).” 🏠 Salt vs phone: the kitchen shrugs, the queue forms. 🔁 Wherever a question gives two points, the midpoint method earns its name first.

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Frequently Asked Questions

Which elasticity do exams ask most?

PED — usually via the total outlay test or a two-point computation. Master Numericals 1 and 2 and you have covered the two standard formats.

Is a negative PED wrong?

No — by the law of demand PED is negative for normal goods; exams accept the absolute value. Positive PED signals an exception (Giffen or Veblen), not a calculation error.

Why did my two methods give two different answers?

They measure against different bases: original value vs midpoint. Both are correct methods; the question’s phrasing (“from ₹20 to ₹24”) selects the midpoint, and naming your method protects the marks.

Does the total outlay test work for supply or income?

No — it is a demand-side test with price as the driver. For income changes, compare budget shares instead (necessities’ share falls as income rises — Engel’s idea, a good bonus point).

How is elasticity used beyond exams?

Pricing (mark-ups are highest where demand is inelastic — the logic Marketing Management Part 4 applies), tax design (the 40% sin slab), and subsidy targeting (the ₹300 Ujjwala DBT reduces the effective price for sensitive households).

Revision One-Liners for the Last Week

1. Elasticity = responsiveness in percentages, not rupees.
2. |PED| > 1 elastic; < 1 inelastic; the five degrees have five shapes.
3. Straight-line demand: elastic top, unitary middle, inelastic bottom.
4. Two points given → midpoint method, named in your answer.
5. Outlay test: price and spending moving together = inelastic.
6. YED > 1 luxury, 0–1 necessity, < 0 inferior; Giffen is the extreme

inferior.

1. XED positive substitutes, negative complements — the market-definition

tool.

1. PES rises momentary → short → long run.
2. Tax burden rests on the less elastic side; sin taxes exploit inelastic

demand.

1. Onion = inelastic demand + supply shocks = policy theatre, 2023–2026

edition.

Active Recall Drill: Six Blanks Before You Sleep

1. PED = ______ ÷ ______. (%ΔQ ÷ %ΔP)
2. Price falls and total outlay falls: demand is ______. (elastic)
3. Tea price ↑, coffee demand ↑: XED sign is ______. (positive)
4. The midpoint method divides each change by the ______ of the two values.

(average)

1. Goods with YED between 0 and 1 are called ______. (necessities)
2. When demand is inelastic, a tax burden falls mainly on ______. (buyers)

← Part 1: Demand and Supply — The Equilibrium Dance Next in the series: production and cost, where supply’s stubbornness gets its own measure.

Read more: Business Economics Part 1: Demand and Supply · Learn more: Marketing Part 4: Pricing and the Elasticity Link

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Written and maintained by the Hmmnm Editorial Team — exam mentors and subject editors. Every card is compiled against the official sources listed and re-checked each cycle.

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