PRICE ELASTICITY OF DEMAND

The quantity demanded of a good is affected by changes in the price of the
good, changes in prices of other goods, changes in income and changes in
other relevant factors. Elasticity is a measure of just how much the quantity
demanded will be affected by a change in price, income, price of other goods
etc..


If the price of steak increases by 1% and the quantity demanded then falls by
20% we can see there has been a very large drop in the amount demanded in
comparison to the change in price. The price elasticity of demand for steak is
said to be high.


If the quantity of steak demanded was to only fall by 0.01%, we can see this is
a fairly insignificant fall in quantity in response to the 1% increase in price. In
this case the price elasticity of demand for steak is low.

It can be calculated using the following formula:



percentage change in quantity demanded/percentage change in price


(To help you remember quantity is on top of price think of the football team
QPR).The table below shows a number of calculations of price elasticity of
demand.

            % change in price     % change in quantity      elasticity 

                    10                                  20                                2   

                    50                                  25                               0.5 

                     7                                   28                                4 

                     9                                    3                                0.33 

Elasticity figure are actually negative, but economists forget this point in the 

name of simplicity.  

ELASTIC AND INELASTIC DEMAND

Different values of price elasticity are given special names:

•  Demand is price elastic, if the value of elasticity is greater than one. If 

demand for a good is price elastic then a percentage change in price 

will lead to an even larger percentage change in the quantity 

demanded. For example if a 10% rise in the price of CDs leads to a 

20% fall in the demand, then price elasticity is 20% / 10% or 2 and the 

demand for CDs is therefore elastic.

SPECIAL CASES OF ELASTICITY

•  Demand is infinitely inelastic if the value of elasticity is zero (zero 

divided by any number). Any change in price would have no effect on 

the quantity demanded.

•  Demand has unitary elasticity if the value of elasticity is exactly 1. This 

means that a percentage change in the price of a good will lead to an 

exact and opposite change in the quantity demanded. For example a 

good would have unitary elasticity if a 10% increase led to a 10% fall in 

the quantity demanded.

•  Demand is infinitely elastic if the value of elasticity is infinity (any 

number divided by zero). A fall in price would lead to an infinite 

increase in quantity demanded (i.e. increasing from zero), whilst an 

increase in price would lead to the quantity demanded falling to zero

The case of unitary elasticity is the curve (known as a rectangular hyperbola).
The perfectly inelastic curve looks like an I and the perfectly elastic curve
looks like an E (without the top!).


Knowing these special cases it makes it easier to spot whether a demand
curve is relatively elastic or inelastic. The demand curve on the left is
relatively elastic (as it looks more like the E) and the demand in the centre is
relatively inelastic (as it looks more like an I).

CHANGES IN ELASTICITY ALONG THE DEMAND CURVE

We mentioned earlier that a good is infinitely elastic if a fall in price leads to 

an infinite rise in quantity. This must occur if quantity was previously zero and 

rises to in response to a fall in price - this can be seen at the top of the 

demand curve.

The opposite occurs at the bottom of the demand curve leading to an 

elasticity of zero.

Also shown on the diagram is the point where elasticity is unitary (equal to 

one), this by definition occurs exactly halfway along the demand curve.

If elasticity is infinite where the demand curve crosses the price axis, but is 

equal to zero when it crosses the quantity axis, then elasticity must change as 

you move along the demand curve. Demand is price inelastic if it has a value 

less than 1 and elastic if greater than 1, these regions are shown above.

IMPORTANCE OF ELASTICITY FOR A BUSINESS

•  If the business is producing on the price elastic section of the demand 

curve, a small percentage change in price leads to a large percentage 

change in quantity demanded. Lowering the price will have the effect of 

increasing total revenue and raising the price will decrease total 

revenue, e.g., if the price of Mars Bars increased by 25% ceteris 

paribus, we would expect their sales to fall dramatically as consumers 

shift to other chocolate bars. This would have the effect of reducing 

their total revenue.

•  If the business is producing on the unitary price elasticity section of the 

demand curve, small changes in price do not change total revenue as 

a percentage change in price will be exactly offset by an inverse 

change in quantity.

•  If the business is producing on the price inelastic section of the 

demand curve, a small percentage change in price leads to a small 

percentage change in quantity demanded. This will have the effect of 

decreasing total revenue when the price is increased and increasing 

total revenue when the price falls. For example if a firm invented a 

miracle cure for the common cold and decided upon a price of 50p a 

pack. The firm sold 10 million packs in the first year of sales. Next year 

they decide to raise prices by 25% and sales fall to 9 million (10% fall), 

the level of sales have dropped, but the total revenue has increased.

It is important to note that the revenue maximising level of production occurs 

when elasticity is unitary, but this isn't necessarily the level where profit is 

maximised. We don't know the firm's costs at different levels of output. 

Furthermore elasticities are notoriously difficult to calculate and errors in the 

elasticity figures could lead to incorrect pricing decisions. 

FACTORS AFFECTING THE PRICE ELASTICITY OF DEMAND

Two factors are usually highlighted by economists:

•  The availability of substitutes. If a product has many substitutes then its 

price elasticity is likely to be high. An increase in price will lead to 

consumers shifting demand to one of its many substitutes (e.g., 

chocolate bars). However if the good has few substitutes, consumers 

will find it harder to replace that good, so its price elasticity is likely to 

be low (e.g. salt).The more widely a product is defined the fewer 

substitutes it is likely to have. Spaghetti has many substitutes, but food 

has none.

•  Time. The longer the period of time, the more price elastic is the 

demand for the product. For example if the price of leaded petrol was 

to increase by 50% my demand for it would not change in the shirt run. 

However as time goes on I would change my car to one that used 

unleaded petrol, therefore in the longrun elasticity becomes greater.