Written by: Edmund Quek

CHAPTER 3
ELASTICITY OF DEMAND AND SUPPLY

LECTURE OUTLINE
1

INTRODUCTION

2

PRICE ELASTICITY O...
Written by: Edmund Quek

1

INTRODUCTION

We have learnt that a fall in price will lead to a rise in quantity demanded and...
Written by: Edmund Quek

If the PED for a good is zero, the demand is perfectly price inelastic which means that a
change ...
Written by: Edmund Quek

In the above diagram, area C represents the gain in revenue resulting from the rise in the
price ...
Written by: Edmund Quek

In the above table, as quantity rises, TR rises from zero to nine and then falls back to zero.
MR...
Written by: Edmund Quek

2.5

Determinants of price elasticity of demand

Number of substitutes
The larger the number of s...
Written by: Edmund Quek

3.2

Interpreting income elasticity of demand

If the YED for a good is positive, the good is a n...
Written by: Edmund Quek

3.4

Determinants of income elasticity of demand

The YED for a good will be lower the higher the...
Written by: Edmund Quek

If the XEDAB is negative, good A and good B are complements, which means that the two
goods are c...
Written by: Edmund Quek

5

PRICE ELASTICITY OF SUPPLY

5.1

Definition and formula of price elasticity of supply

Definit...
Written by: Edmund Quek

supply curve which intersects the origin has a unit price elasticity of supply at all points
alon...
Written by: Edmund Quek

Behaviour of marginal cost as firms change output
Given a change in price of a good, the lower th...
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Chapter 3-elasticity-of-demand-and-supply

  1. 1. Written by: Edmund Quek CHAPTER 3 ELASTICITY OF DEMAND AND SUPPLY LECTURE OUTLINE 1 INTRODUCTION 2 PRICE ELASTICITY OF DEMAND 2.1 2.2 2.3 2.4 2.5 Definition and formula of price elasticity of demand Interpreting price elasticity of demand Use of price elasticity of demand Linear downward-sloping demand curve (optional) Determinants of price elasticity of demand 3 INCOME ELASTICITY OF DEMAND 3.1 3.2 3.3 3.4 Definition and formula of income elasticity of demand Interpreting income elasticity of demand Use of income elasticity of demand Determinants of income elasticity of demand 4 CROSS ELASTICITY OF DEMAND 4.1 4.2 4.3 4.4 Definition and formula of cross elasticity of demand Interpreting cross elasticity of demand Use of cross elasticity of demand Determinants of cross elasticity of demand 5 PRICE ELASTICITY OF SUPPLY 5.1 5.2 5.3 5.4 Definition and formula of price elasticity of supply Interpreting price elasticity of supply Linear upward-sloping supply curve (optional) Determinants of price elasticity of supply 6 LIMITATIONS OF THE CONCEPTS OF ELASTICITY OF DEMAND References John Sloman, Economics William A. McEachern, Economics Richard G. Lipsey and K. Alec Chrystal, Positive Economics G. F. Stanlake and Susan Grant, Introductory Economics Michael Parkin, Economics David Begg, Stanley Fischer and Rudiger Dornbusch, Economics © 2011 Economics Cafe All rights reserved. Page 1
  2. 2. Written by: Edmund Quek 1 INTRODUCTION We have learnt that a fall in price will lead to a rise in quantity demanded and vice versa. However, given any change in price, in addition to the direction of the change in quantity demanded, economists are also interested to find the magnitude of the change. To measure this, economists use the concept of price elasticity of demand. This chapter gives an exposition of the concepts of price elasticity of demand, income elasticity of demand, cross elasticity of demand and price elasticity of supply. 2 PRICE ELASTICITY OF DEMAND 2.1 Definition and formula of price elasticity of demand Definition The price elasticity of demand (PED) for a good is a measure of the degree of responsiveness of the quantity demanded to a change in the price, ceteris paribus. Formula %  Quantity Demanded PED  ------------------------------%  Price 2.2 Interpreting price elasticity of demand Due to the law of demand, the PED for a good is always negative. However, the common practice among economists is to omit the negative sign. If the PED for a good is greater than one, the demand is price elastic which means that a change in the price will lead to a larger percentage change in the quantity demanded. A good with an elastic demand has a relatively flat demand curve. If the PED for a good is less than one, the demand is price inelastic which means that a change in the price will lead to a smaller percentage change in the quantity demanded. A good with an inelastic demand has a relatively steep demand curve. If the PED for a good is equal to one, the demand is unit price elastic which means that a change in the price will lead to the same percentage change in the quantity demanded. The demand curve for a good with a unit elastic demand is a rectangular hyperbola. © 2011 Economics Cafe All rights reserved. Page 2
  3. 3. Written by: Edmund Quek If the PED for a good is zero, the demand is perfectly price inelastic which means that a change in the price will not lead to any change in the quantity demanded. A good with a perfectly inelastic demand has a vertical demand curve. If the PED for a good is infinity, the demand is perfectly price elastic which means that a rise (not a fall) in the price will lead to an infinite decrease in the quantity demanded. A good with a perfectly elastic demand has a horizontal demand curve. 2.3 Use of price elasticity of demand If the demand for a good is price elastic, a firm can increase total revenue by lowering the price as the quantity demanded will increase by a larger percentage. In the above diagram, area A represents the gain in revenue resulting from the increase in the quantity demanded (Q) and area B represents the loss in revenue resulting from the fall in the price (P). Since the gain exceeds the loss, total revenue rises. If the demand for a good is price inelastic, a firm can increase total revenue by raising the price as the quantity demanded will decrease by a smaller percentage. © 2011 Economics Cafe All rights reserved. Page 3
  4. 4. Written by: Edmund Quek In the above diagram, area C represents the gain in revenue resulting from the rise in the price (P) and area D represents the loss in revenue resulting from the decrease in the quantity demanded (Q). Since the gain exceeds the loss, total revenue rises. If the demand for a good is unit price elastic, a firm cannot increase total revenue by changing the price as the quantity demanded will change by the same percentage. 2.4 Linear downward-sloping demand curve (optional) Along a linear downward-sloping demand curve, PED is different at different points along the demand curve. As we move down along a linear downward-sloping demand curve, PED falls from infinity to zero. If the demand curve is linear, the total revenue curve will be Hill-shaped. Example Price 6 5 4 3 2 1 0 Quantity demanded 0 1 2 3 4 5 6 Total revenue 0 5 8 9 8 5 0 Average revenue 5 4 3 2 1 0 Marginal revenue 5 3 1 -1 -3 -5 PED ∞ 5 2 1 0.5 0.2 0 Total revenue (TR)  Price (P) x Quantity (Q) Average revenue (AR)  TR/Q  P (∴ Demand curve  AR curve) Marginal revenue (MR)  ΔTR/ΔQ © 2011 Economics Cafe All rights reserved. Page 4
  5. 5. Written by: Edmund Quek In the above table, as quantity rises, TR rises from zero to nine and then falls back to zero. MR is the additional revenue resulting from selling one more unit of a good. TR is maximised where PED is 1 since any change in price will lead to the same percentage change in quantity demanded. If we assume that quantity is continuously divisible, this will happen where MR is equal to zero. TR curve and MR curve that correspond to a linear downward-sloping demand curve Note: MR is lower than AR because if the firm wants to sell one more unit of the good, not only must it lower the price of the unit, but it must also lower the price of all the previous units. With the use of differential calculus, we can show that the slope of the MR curve is twice that of the AR curve. When economists speak of elastic demand, what they mean is that PED is greater than one at all the points along the demand curve and a necessary condition for this to happen is that the demand curve is not linear. © 2011 Economics Cafe All rights reserved. Page 5
  6. 6. Written by: Edmund Quek 2.5 Determinants of price elasticity of demand Number of substitutes The larger the number of substitutes for a good, the higher the PED for the good. Conversely, the smaller the number of substitutes for a good, the lower the PED for the good. The number of substitutes for a good depends, in part, on how narrowly, and for that matter, how broadly the good is defined. The more narrowly a good is defined (e.g. carrots, cabbages), the more elastic the demand for the good. Conversely, the more broadly a good is defined (e.g. vegetables or even food), the less elastic the demand for the good. Closeness of substitutes The closer the substitutes for a good, the higher the PED for the good. Conversely, the farther the substitutes for a good, the lower the PED for the good. Degree of necessity The higher the degree of necessity of a good, the lower the PED for the good. Conversely, the lower the degree of necessity of a good, the higher the PED for the good. Proportion of income spent on the good The larger the proportion of income spent on the good, the higher the PED for the good. Conversely, the smaller the proportion of income spent on a good, the lower the PED for the good. Time period The longer the time period after a change in the price of a good, the higher the PED for the good. 3 INCOME ELASTICITY OF DEMAND 3.1 Definition and formula of income elasticity of demand Definition The income elasticity of demand (YED) for a good is a measure of the degree of responsiveness of the quantity demanded to a change in income, ceteris paribus. Formula %  Quantity Demanded YED  -------------------------------%  Income © 2011 Economics Cafe All rights reserved. Page 6
  7. 7. Written by: Edmund Quek 3.2 Interpreting income elasticity of demand If the YED for a good is positive, the good is a normal good. A normal good is a good whose demand rises when consumers’ income rises. Normal goods can be classified into two types. A normal good that is income elastic is known as a luxury and it has a YED greater than one. A normal good that is income inelastic is known as a necessity and it has a YED less than one. If the YED for a good is negative, the good is an inferior good. An inferior good is a good whose demand falls when consumers’ income rises. The concepts of normal and inferior goods can be depicted by an Engel’s curve. The Engel’s curve of a good shows the quantity of the good demanded at each income level. The Engel’s curve In the above diagram, the good is normal between income levels Y0 and Y1 and inferior between income levels Y1 and Y2. 3.3 Use of income elasticity of demand The concept of YED allows a firm to determine the future size of the market for its good and hence its production capacity. Suppose that the YED for a good is positive. If a firm that produces the good predicts an economic boom, it can consider increasing its production capacity to meet the increase in demand if the boom arrives. Conversely, if the firm predicts a recession, it can consider decreasing its production capacity or holding back any expansion plan to minimise excess production capacity if the recession arrives. © 2011 Economics Cafe All rights reserved. Page 7
  8. 8. Written by: Edmund Quek 3.4 Determinants of income elasticity of demand The YED for a good will be lower the higher the level of income and wealth of consumers. 4 CROSS ELASTICITY OF DEMAND 4.1 Definition and formula of cross elasticity of demand Definition The cross elasticity of demand (XED) for a good with respect to another good is a measure of the degree of responsiveness of the quantity of the first good demanded to a change in the price of the second good, ceteris paribus. Let the two goods be good A and good B. Formula %  Quantity Demanded of Good A XEDAB = ----------------------------------------------%  Price of Good B 4.2 Interpreting cross elasticity of demand If the XEDAB is positive, good A and good B are substitutes, which means that the two goods are alternatives to each other. In the above diagram, the demand curve (DAB) relating the quantity demanded of good A to the price of good B is upward-sloping. If the price of good B rises, consumers will buy less of it. Since good A and good B are substitutes, they will buy more good A. © 2011 Economics Cafe All rights reserved. Page 8
  9. 9. Written by: Edmund Quek If the XEDAB is negative, good A and good B are complements, which means that the two goods are consumed together. In the above diagram, the demand curve (DAB) relating the quantity demanded of good A to the price of good B is downward-sloping. If the price of good B rises, consumers will buy less of it. Since good A and good B are complements, they will buy less good A. 4.3 Use of cross elasticity of demand The concept of XED allows a firm to determine how a change in the price of a related good produced by another firm will affect the demand for its good. For instance, if a firm that produces a substitute decreases its price, the firm will also need to decrease its price to avoid suffering a decrease in demand and this is due to the positive XED between substitutes. However, it should take into consideration the possibility of a price war. If a firm that produces a substitute increases its price, the firm can increase total revenue by keeping its price unchanged. However, if does not have excess production capacity, it may need to increase its price to increase total revenue. If a firm produces more than one good, let’s say two goods, the concept of XED also allows the firm to determine how the demand for one good will be affected if it changes the price of the other good. 4.4 Determinants of cross elasticity of demand The XED between two goods will be higher the more closely they are related. Hence, a large positive XED indicates very close substitutes and a large negative XED indicates very close complements. © 2011 Economics Cafe All rights reserved. Page 9
  10. 10. Written by: Edmund Quek 5 PRICE ELASTICITY OF SUPPLY 5.1 Definition and formula of price elasticity of supply Definition The price elasticity of supply (PES) of a good is a measure of the degree of responsiveness of the quantity of the good supplied to a change in the price, ceteris paribus. Formula %  Quantity Supplied PES  -----------------------------%  Price 5.2 Interpreting price elasticity of supply Due to the law of supply, the PES of a good is always positive. If the PES of a good is greater than one, the supply of the good is price elastic which means that a change in the price of the good will lead to a larger percentage change in the quantity supplied. A good with an elastic supply has a relatively flat supply curve. If the PES of a good is less than one, the supply of the good is price inelastic which means that a change in the price of the good will lead to a smaller percentage change in the quantity supplied. A good with an inelastic supply has a relatively steep supply curve. If the PES of a good is equal to one, the supply of the good is unit price elastic which means that a change in the price of the good will lead to the same percentage change in the quantity supplied. If the PES of a good is zero, the supply of the good is perfectly price inelastic which means that a change in the price of the good will not lead to any change in the quantity supplied. A good with a perfectly inelastic supply has a vertical supply curve. If the PES of a good is infinity, the supply of the good is perfectly price elastic which means that a fall (not a rise) in the price of the good will lead to an infinite decrease in the quantity supplied. A good with a perfectly elastic supply has a horizontal supply curve. 5.3 Linear upward-sloping supply curve (optional) Along a linear upward-sloping supply curve, PES is different at different points along the supply curve, unless the supply curve intersects the origin as a linear upward-sloping © 2011 Economics Cafe All rights reserved. Page 10
  11. 11. Written by: Edmund Quek supply curve which intersects the origin has a unit price elasticity of supply at all points along the supply curve, regardless of the slope. A linear upward-sloping supply curve which intersects the price-axis has a PES greater than one at all points along the supply curve, regardless of the slope. Although all the points along such a supply curve has a PES greater than one, the value of PES decreases as we move up along the supply curve. A linear upward-sloping supply curve which intersects the quantity-axis has a PES less than one at all points along the supply curve, regardless of the slope. Although all the points along such a supply curve has a PES less than one, the value of PES increases as we move up along the supply curve. 5.4 Determinants of price elasticity of supply Definition of the good The PES of a good is higher the more narrowly the good is defined. For instance, the PES of a crop is higher than the PES of crops as a whole as it is easier to obtain factor inputs to produce a crop within the agricultural sector than from other industries. Time period The longer the time period after an increase in the price of a good, the higher the PES of the good as firms are able to increase output by a larger amount with more time. Time period can be divided into the immediate run, the short run and the long run. The immediate run is the time period that is so short that output is fixed. The supply curve of the good is perfectly inelastic, assuming firms do not keep stocks of the good. The short run is the time period during which at least one of the factor inputs used in the production process is fixed. In the short run, when the price of a good rises, firms can increase output only by employing more of the variable factor inputs used in the production process. The long run is the time period after which all the factor inputs used in the production process are variable. The PES of a good in the long run is higher than the PES in the short run as firms can increase output by employing more of all the factor inputs used in the production process and potential firms can enter the industry in the long run. Nature of the good The supply of a non-perishable good is normally more price elastic than the supply of a perishable good. If a good is non-perishable, such as a manufactured good, firms can keep stocks of the good. Therefore, if the price rises, firms can increase the quantity supplied by increasing output and by running down their stocks. However, if a good is perishable, such as an agricultural product, firms cannot keep stocks of the good. Therefore, if the price rises, firms can only increase the quantity supplied by increasing output. Further, the growing time of a perishable good is normally longer than the production time of a non-perishable good. © 2011 Economics Cafe All rights reserved. Page 11
  12. 12. Written by: Edmund Quek Behaviour of marginal cost as firms change output Given a change in price of a good, the lower the output level and hence capacity utilisation, the less rapidly marginal cost will change as firms change output. The less rapidly marginal cost will change as firms change output, the larger the change in output. Therefore, the lower the output level of a good, the higher the PES of the good. Mobility of factor inputs The ease with which factor inputs can move from the production of one good to another will influence PES. The higher the mobility of factor inputs, the higher the PES of a good. The mobility of factor inputs depends to some extent on the time period. 6 LIMITATIONS OF THE CONCEPTS OF ELASTICITY OF DEMAND The concepts of elasticity of demand are subject to several limitations. - Data from past records (secondary data) may no longer be relevant because some of the non-price determinants of demand may have changed. - Data from current surveys (primary data) may not be reliable because the respondents might not have been truthful about the answers that they gave. - If the sample size is small, the results of the surveys may not be reflective of the actual market for the good. However, with data that are reasonably accurate, the concepts of elasticity of demand can be useful to a firm for making business decisions. Note: More advanced applications of the concepts of elasticity of demand will be discussed in the essays. In addition, students must also know the usefulness of the concepts of elasticity of demand to the government which will also be discussed in the essays. © 2011 Economics Cafe All rights reserved. Page 12

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