Income Elasticity measures the percentage change in quantity demanded relative to a percentage change in consumer income.
## Core concept
Income Elasticity of Demand (YED) tells you how responsive consumer demand is to changes in income. It is calculated at a specific point on the demand curve and helps classify goods as normal or inferior.
- Formula: YED = (% change in quantity demanded) ÷ (% change in income)
- Or: YED = (ΔQ/Q) ÷ (ΔY/Y), where Y = income
- Arc method: YED = [(Q₂ − Q₁) / {(Q₁ + Q₂)/2}] ÷ [(Y₂ − Y₁) / {(Y₁ + Y₂)/2}]
## Classification of goods
| YED Range | Good Type | Behaviour | |-----------|-----------|-----------| | YED > 1 | Luxury/Superior | Demand rises faster than income; spending share increases | | 0 < YED < 1 | Normal (necessity) | Demand rises slower than income; spending share falls | | YED < 0 | Inferior | Demand falls when income rises (e.g. cheap pulses, bus travel) | | YED = 0 | Income-inelastic | Demand unaffected by income change |
## Determinants of income elasticity
- Nature of the good: Luxuries tend to have high YED; essentials have low YED
- Income level of consumer: Low-income consumers may treat some goods as luxuries; high-income consumers treat them as necessities
- Proportion of income spent: Goods consuming a small share of budget tend to have lower YED
- Availability of substitutes: Affects demand patterns as income changes
## Common exam applications
- Business forecasting: If income rises 5% and YED for a product is 2, demand will increase by 10%
- Production planning: Firms producing luxuries benefit during economic booms; inferior good producers benefit during recessions
- Policy analysis: Understanding whether a good is normal or inferior helps predict welfare effects of income redistribution
- Market segmentation: Different income groups have different YED for the same good
## Worked example
A consumer buys 20 units of coffee at an annual income of ₹30,000. When income rises to ₹36,000, quantity demanded increases to 24 units.
Calculation (point method): - % change in Q = (24 − 20) / 20 × 100 = 20% - % change in Y = (36,000 − 30,000) / 30,000 × 100 = 20% - YED = 20% ÷ 20% = 1 (unit elastic, normal necessity)
This means a 1% increase in income leads to a 1% increase in coffee demand.
## Common mistakes
- Confusing YED with PED: Income elasticity uses income changes; price elasticity uses price changes (Topic: Price Elasticity)
- Ignoring the sign: YED < 0 is still elastic/inelastic based on absolute value; the sign only indicates good type
- Assuming all goods are normal: Backward-sloping demand for inferior goods is income-driven, not price-driven
- Using wrong denominator in percentage: Always divide change by the original value (or mid-point for arc method), not the new value
- Forgetting context: Same good can shift between normal and inferior status across different income groups or time periods
## Exam-focused tip
When a question gives income and quantity data for two scenarios, always calculate YED using the arc method unless told otherwise. Classify the good immediately. Then explain business implications (stock levels, marketing strategy, forecasting). This three-step approach covers most CA Foundation exam questions on this topic.