## AS Economics – Price Elasticity of Demand

Doing some revision courses for AS students and went over Price Elasticity of Demand. Might be useful for those doing AS at the moment.

Price Elasticity of Demand (PED)
This measures the relative amount by which the quantity demanded will change in response to change in the price of a particular good. The equation is:

% change in Quantity ÷ Demanded % change in Price

How is PED calculated?

Consider the following demand schedule for buses in a city centre.

Price (average fare)          Quantity of passengers per week
100c                                      1000
60c                                        1300
30c                                        2275

Suppose the current average fare was 100c, what is the PED if fares are cut to 60c?

The percentage change in QD is equal to:
• The change in demand 300 (1300-1000) divided by the original level of demand 1000. To obtain a percentage this must be multiplied by 100. The full calculation is (300 ÷ 1000) x 100 = 30%

The percentage change in price is equal to:
• The change in price 40c (100c – 60c) divided by the original price 100c. To obtain a percentage this must be multiplied by 100. The full calculation is (40 ÷ 100) x 100 = 40%

These two figures can then be inserted into the formula with 30% ÷ 40% = 0.75
Let us now consider the PED when the average fare is cut from 60c to 30c

The percentage change in QD is equal to:
• The change in demand 975 (2275-1300) divided by the original level of demand 1300. To obtain a percentage this must be multiplied by 100. The full calculation is (975 ÷ 1300) x 100 = 75%

The percentage change in price is equal to:
• The change in price 30c (60c – 30c) divided by the original price 60c. To obtain a percentage this must be multiplied by 100. The full calculation is (30 ÷ 60) x 100 = 50%

These two figures can then be inserted into the formula with 75% ÷ 50% = 1.5

Please note that the minus sign is often omitted in PED, as the price elasticity is always negative because demand curves slope downwards. The textbook displays figures as:
PED = (-) 0.2

What price elasticity of demand figures tell us.

Determinants of Elasticity of Demand

The elasticity of a product is influenced by:
• the number of substitutes available
• whether it could be described as a luxury or a basic commodity
• the proportion of the purchaser’s income it represents
• the durability of the product.

Usefulness of Price Elasticity of Demand

The usefulness of price elasticity for producers. Firms can use price elasticity of demand (PED) estimates to predict:

1. The effect of a change in price on the total revenue & expenditure on a product.

The relationship between elasticity and total revenue.

Elastic         Inelastic            Unitary
Price ↑           TR↓                TR↑                      No Change
Price ↓           TR↑                TR↓                      No Change

2. The likely price volatility in a market following unexpected changes in supply.

3. The effect of a change in GST (indirect tax) on price and quantity demanded and also whether the business is able to pass on some or all of the tax onto the consumer.

4. Information on the price elasticity of demand can be used by a business as part of a policy of price discrimination – off-peak and peak travel in major cities. Before 9am – inelastic demand curve – after 9am elastic demand curve.

## Cobweb Theory and Price Elasticity

I have blogged on this topic before and although not in the NCEA or CIE syllabus’ I find it useful theory to mention when doing supply, demand and elasticity. Agricultural markets are particularly vulnerable to price fluctuations. many agricultural products have inelastic demand and inelastic supply. This means that any change in demand or supply has more of an impact on price than on quantity. Price fluctuations can also arise due to the time lag between planning agricultural production and selling the produce. The cobweb theory (so-called because of the appearance of the diagram) suggests that price can fluctuate around the equilibrium for some time, or even move away from the equilibrium. Dairy farmers base their production decisions on the price prevailing in the previous time period.

The supply of dairy products in New Zealand fits this assumption – farmers make their production decisions today, but the dairy cooperatives (Fonterra, Westland, etc.) don’t make a final decision on the price farmers will receive until close to the end of the season.

Cobweb scenarios:
Convergent
At the equilibrium point, if the demand curve is more elastic than the supply curve, we get the price volatility falling, and the price will converge on the equilibrium. For example:

• Adverse weather conditions means their is a poor crop – Qt
• The excess demand causes the price to rise – Pt
• Because of the higher price famers plant more crops and therefore greater supply – Qt+1
• With supply so high prices drop to meet demand – Pt+1
• Lower prices mean that famers supply less to the following year – Qt+2
• This results in higher prices again – Pt+2
• Because of the higher price famers plant more crops and therefore greater supply – Qt+3 etc.
• This process continues until you get to an equilibrium as the PED is greater than the PES – supply curve is steeper than the demand curve.