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Elasticity . . .弾力性とは

• … allows us to analyze supply and demandwith greater precision.

• … is a measure of how much buyers and sellersrespond to changes in market conditions

• 定義：弾力性は供給と需要の変動を正確


• 定義：弾力性は供給と需要の変動を正確に分析する方法であり、市場において売り手と買い手は価格変化にどれだけ反応するかを測定する尺度である。

THE ELASTICITY OF DEMAND需要の弾力性

• The price elasticity of demand is a measure of howmuch the quantity demanded of a good responds to achange in the price of that good. When we talk aboutelasticity, that responsiveness is always measured inpercentage terms. Specifically, the price elasticity of


percentage terms. Specifically, the price elasticity ofdemand is the percentage change in quantitydemanded due to a percentage change in the price.

定義：需要の価格弾力性はある商品の価格の変化により需要の量はどれだけ変化するかを測る尺度である。百分率で表示する。

The Price Elasticity of Demand and ItsDeterminants需要の価格弾力性とその決定要因

• Availability of Close Substitutes

密接な代替財の有無

• Necessities versus Luxuries

非需品と贅沢品


非需品と贅沢品

• Definition of the Market

市場の定義（競争市場か、代替財の多少など）

• Time Horizon

時間の推移

The Price Elasticity of Demand and ItsDeterminants需要の価格弾力性とその決定要因

• Demand tends to be more elastic:

需要はもっと弾力的、

• the larger the number of close substitutes.代替品は多ければ


多ければ

• if the good is a luxury.贅沢品であれば

• the more narrowly defined the market. 市場の定義は狭い場合は

• the longer the time period. 長期間であれば

Computing the Price Elasticity of Demand需要の価格弾力性を計算してみよう

• The price elasticity of demand is computed as the percentagechange in the quantity demanded divided by the percentagechange in price.

需要の価格弾力性は需要量変化の百分率割る価格変化の百分率。

需要量変化率（％）


需要量変化率（％）

需要の価格弾力性＝－－－－－－－－－－－

価格変化率（％）

Price elasticity of demand =Percentage change in quantity demanded

Percentage change in price

Computing the Price Elasticity of Demand需要の価格弾力性を計算してみよう

• Example: If the price of an ice cream cone increases from $2.00to $2.20 and the amount you buy falls from 10 to 8 cones, thenyour elasticity of demand would be calculated as:

• 例：アイスクリームの価格は2ドルから2.2ドルまで上昇したら、消費量は１０個から８個まで減少した。

需要の価格弾力性の計算は以下のとおり。


( )

( . . )

.

10 8

10100

2 20 2 00

2 00100

20%

10%2

Price elasticity of demand =Percentage change in quantity demanded


The Midpoint Method: A Better Way to CalculatePercentage Changes and Elasticities中間点方法：より正確的に弾力性を測る

• The midpoint formula is preferable whencalculating the price elasticity of demandbecause it gives the same answer regardless ofthe direction of the price change.


the direction of the price change.

2 1 2 1

2 1 2 1

( ) /[( ) / 2]Price elasticity of demand =

( ) /[( ) / 2]

Q Q Q Q

P P P P

The Midpoint Method: A Better Way to CalculatePercentage Changes and Elasticities中間点方法：より正確的に弾力性を測る

• Example: If the price of an ice cream coneincreases from $2.00 to $2.20 and the amountyou buy falls from 10 to 8 cones, then yourelasticity of demand, using the midpoint


elasticity of demand, using the midpointformula, would be calculated as:

(10 8)

22%(10 8)/22.32

(2.20 2.00) 9.5%(2.00 2.20)/2

The Variety of Demand Curvesさまざまな需要曲線

• Inelastic Demand 非弾力的

• Quantity demanded does not respond strongly toprice changes. Price elasticity of demand is lessthan one.

• 需要の量は価格の変化に反応なし。需要の弾力


• 需要の量は価格の変化に反応なし。需要の弾力性は１より小さい。E<１

• Elastic Demand 弾力的

• Quantity demanded responds strongly to changes inprice. Price elasticity of demand is greater than one.

• 需要の量は価格の変化に反応が大きい。需要の弾力性は１より大きい。 E>１

Computing the Price Elasticity of Demand価格弾力性の計算例

$5

5.00)/2(4.005.00)(4.00

50)/2(10050)(100

ED

Price


Demand is price elastic.

4Demand

Quantity1000 50

3percent22

percent67


• Perfectly Inelastic 完全非弾力的

• Quantity demanded does not respond to price changes.需要の量は完全に価格の変化に反応なし。

• Perfectly Elastic 完全弾力的


• Quantity demanded changes infinitely with any changein price. 需要の量は価格の変化に無限に反応。

• Unit Elastic 単位弾力的

• Quantity demanded changes by the same percentage asthe price.

• 需要の量は価格の変化に同じ百分率で反応。


• Because the price elasticity of demandmeasures how much quantity demandedresponds to the price, it is closely related to theslope of the demand curve.


slope of the demand curve.

• But it is not the same thing as the slope!

• 需要の価格弾力性は需要曲線の傾きと似ているが、同じものではない。

Figure 1 The Price Elasticity of Demand完全に非弾力的

(a) Perfectly Inelastic Demand: Elasticity Equals 0

$5

DemandPrice


4

Quantity1000

1. Anincreasein price . . .

2. . . . leaves the quantity demanded unchanged.

Figure 1 The Price Elasticity of Demand非弾力的：弾力性は１より小さい

(b) Inelastic Demand: Elasticity Is Less Than 1

$5

Price


Quantity0 90

Demand1. A 22%increasein price . . .

2. . . . leads to an 11% decrease in quantity demanded.

4

100

Figure 1 The Price Elasticity of Demand単位弾力的：弾力性は１

(c) Unit Elastic Demand: Elasticity Equals 1

Price

$5


2. . . . leads to a 22% decrease in quantity demanded.

Quantity

4

1000 80

1. A 22%increasein price . . .

Demand

Figure 1 The Price Elasticity of Demand弾力的：弾力性は１より大きい

(d) Elastic Demand: Elasticity Is Greater Than 1

Demand4

Price

$5


Demand

Quantity

4

1000 50


2. . . . leads to a 67% decrease in quantity demanded.

Figure 1 The Price Elasticity of Demand完全弾力的：弾力性は無限大

(e) Perfectly Elastic Demand: Elasticity Equals Infinity

Price

$4 Demand

1. At any priceabove $4, quantitydemanded is zero.


Quantity0

$4 Demand

2. At exactly $4,consumers willbuy any quantity.

3. At a price below $4,quantity demanded is infinite.

Total Revenue and the Price Elasticity ofDemand 総収入と需要の弾力性

• Total revenue is the amount paid by buyers andreceived by sellers of a good.

• Computed as the price of the good times the quantitysold.

• 総収入は商品の価格かける販売量。


• 総収入は商品の価格かける販売量。

Figure 2 Total Revenue総収入曲線

Price

$4

When the price is $4, consumerswill demand 100 units, and spend$400 on this good.


Demand

Quantity

Q

P

0

P × Q = $400(revenue)

$4

100

Elasticity and Total Revenue along a LinearDemand Curve線形需要曲線における総収入と弾力性

• With an inelastic demand curve, an increase inprice leads to a decrease in quantity that isproportionately smaller. Thus, total revenueincreases.


increases.

Figure 3 How Total Revenue Changes When Price Changes:Inelastic Demand価格変化の場合は総収入はどのように変化するか：非弾力的

Price Price

An Increase in price from $1to $3 …

… leads to an Increase intotal revenue from $100 to$240


Demand

Quantity0

Revenue = $100

Quantity0

Revenue = $240

Demand$1

100

$3

80

Elasticity and Total Revenue along a Linear DemandCurve

• With an elastic demand curve, an increase inthe price leads to a decrease in quantitydemanded that is proportionately larger. Thus,total revenue decreases.


total revenue decreases.

Figure 3 How Total Revenue Changes When Price Changes:Elastic Demand価格変化の場合は総収入はどのように変化するか：弾力的

Price Price

$5

An Increase in price from $4to $5 …

… leads to an decrease intotal revenue from $200 to$100


Demand

Quantity0

Revenue = $200

$4

50

Demand

Quantity0

Revenue = $100

20

Note that with each price increase, the Law of Demand still holds – anincrease in price leads to a decrease in the quantity demanded. It is thechange in TR that varies!

Elasticity of a Linear Demand Curve線形需要曲線の弾力性（注意：変化率で判断！）


4

5

6

$7

Demand is elastic;demand is responsive tochanges in price.

When price increases from$4 to $5, TR declines from$24 (=4×6) to $20 (=5×4).

浅青色の面積

Elasticity is > 1 in this range.

Elasticity is < 1 in this range.

Price

Figure 4 Elasticity of a Linear Demand Curve線形需要曲線の弾力性


0 2 64 108 12 14

2

1

4

3Demand is inelastic; demand isnot very responsive to changesin price.

When price increases from $2 to $3,TR increases from $20 (=2×10) to$24 (=3×8).黄色の面積

Elasticity is < 1 in this range.

Quantity

Other Demand Elasticitiesそのたの弾力性

• Income Elasticity of Demand 所得の弾力性

• Income elasticity of demand measures how muchthe quantity demanded of a good responds to achange in consumers’ income.


• It is computed as the percentage change in thequantity demanded divided by the percentagechange in income.

需要量の変化率割る所得の変化率

Other Demand Elasticitiesその他の弾力性

• Computing Income Elasticity 所得弾力性の計算


Remember, all elasticities are measured bydividing one percentage change by another.

注意：すべての弾力性は変化率（％）で計算する。


• Income Elasticity 所得の弾力性

• Types of Goods 商品の種類

• Normal Goods 正常財

• Inferior Goods 劣等財


• Higher income raises the quantity demanded fornormal goods but lowers the quantity demanded forinferior goods.

トレンド：所得は向上すると、正常財への需要は増加、劣等財への需要は減少。


• Income Elasticity 所得の弾力性

• Goods consumers regard as necessities tend to beincome inelastic 非需品の所得弾力性は小さい

• Examples include food, fuel, clothing, utilities, andmedical services. 例：食料、石油、衣服、光熱費、医療


medical services. 例：食料、石油、衣服、光熱費、医療サービス。

• Goods consumers regard as luxuries tend to beincome elastic. 贅沢品の所得弾力性は大きい。

• Examples include sports cars, furs, and expensive foods.

• 例：スポーツカー、革製品、高い食品


• Cross-price elasticity of demand 需要の交差弾力性• A measure of how much the quantity demanded of one good

responds to a change in the price of another good, computedas the percentage change in quantity demanded of the firstgood divided by the percentage change in the price of the


good divided by the percentage change in the price of thesecond good

THE ELASTICITY OF SUPPLY供給の弾力性• Price elasticity of supply is a measure of how

much the quantity supplied of a good respondsto a change in the price of that good.

• Price elasticity of supply is the percentage


• Price elasticity of supply is the percentagechange in quantity supplied resulting from apercentage change in price.

• 供給の弾力性とは、価格の変化に供給量の変化率を量る尺度である。同じく百分率で表示する。

Figure 5 The Price Elasticity of Supply 供給の弾力性(a) Perfectly Inelastic Supply: Elasticity Equals 0

完全非弾力的：E＝０

$5

4

SupplyPrice


4

Quantity1000

1. Anincreasein price . . .

2. . . . leaves the quantity supplied unchanged.

Figure 5 The Price Elasticity of Supply 供給の弾力性

(b) Inelastic Supply: Elasticity Is Less Than 1

完全非弾力的：E<１

$5

4

Price

Supply


110100

4

Quantity0


2. . . . leads to a 10% increase in quantity supplied.


(c) Unit Elastic Supply: Elasticity Equals 1

単位弾力性：E＝１

$5

4

Price

Supply

(If SUPPLY is unit


125100

4

Quantity0



(If SUPPLY is unitelastic and linear, it willbegin at the origin.)


(d) Elastic Supply: Elasticity Is Greater Than 1弾力性＞１

Price

4

$5

Supply


Quantity0



4

100 200


(e) Perfectly Elastic Supply: Elasticity Equals Infinity完全弾力的：無限大

Price

1. At any priceabove $4, quantitysupplied is infinite.


Quantity0

$4 Supply

3. At a price below $4,quantity supplied is zero.

2. At exactly $4,producers willsupply any quantity.

The Price Elasticity of Supply and ItsDeterminants 供給の弾力性とその決定要因

• Ability of sellers to change the amount of thegood they produce.売り手の商品の量の変化能力に依存する。

• Beach-front land is inelastic. ビーチに面する土地


• Beach-front land is inelastic. ビーチに面する土地は非弾力的。

• Books, cars, or manufactured goods are elastic. 本、車、工業製品は弾力的。

• Time period 時間の推移

• Supply is more elastic in the long run.

• 長期的に供給は弾力的。

Computing the Price Elasticity of Supply需要の弾力性を測る

• The price elasticity of supply is computed asthe percentage change in the quantity supplieddivided by the percentage change in price.

需要の変化率（％）


• 需要の弾力性＝ー－－－－－－－－－

価格の変化率（％）

Price elasticity of supply =

Percentage changein quantity supplied


THREE APPLICATIONS OF SUPPLY, DEMAND,AND ELASTICITY 供給、需要および弾力性の応用

• Can good news for farming be bad news for farmers?

よい収穫は農家に対して悪いのか。

• What happens to wheat farmers and the market forwheat when university agronomists discover a new


wheat when university agronomists discover a newwheat hybrid that is more productive than existingvarieties?

大学の農学者はより生産的小麦の雑種を開発した場合は、小麦農家に何かの変化が引き起こるのか。

Can Good News for Farming Be Bad News forFarmers?よい収穫は農家に対して悪いのか

• Examine whether the supply or demand curve shifts.

供給曲線と需要曲線の変化を検証しよう。

• Determine the direction of the shift of the curve.

それは各曲線の方向を決めるのだ。


• Use the supply-and-demand diagram to see how themarket equilibrium changes.

供給表と需要表を利用してその変化を見てみよう。

Figure 7 An Increase in Supply in the Market for Wheat小麦市場において生産量の増加：小麦価格の下落

Price of

Wheat

S1 S2

2. . . . leads

to a large fall

in price . . .

1. When demand is inelastic,

an increase in supply . . .

$3


Quantity of

Wheat0

3. . . . and a proportionately smaller

increase in quantity sold. As a result,

revenue falls from $300 to $220.

Demand

2

110100

Compute the Price Elasticity of Demand When There Is aChange in Supply供給の変化により需要の価格弾力性を計算してみよ

ED

100 110

100 110 2300 2 00

300 2 00 2

( ) /. .

( . . ) /


0 095

0 40 24

.

..

Demand is inelastic.

需要は非弾力的

Why Did OPEC Fail to Keep the Price of Oil High?なぜ石油輸出機構は石油価格の向上を維持できないのか

• Supply and Demand can behave differently in the short runand the long run 供給と需要の短期的反応と長期的反応は異なる。

• In the short run, both supply and demand for oil arerelatively inelastic 短期的に石油の供給と需要は非弾力的であるが、


的であるが、

• But in the long run, both are elastic 長期的に両方とも弾力的である。

• Production outside of OPEC OPEC以外の国の石油生産

• More conservation by consumers 消費者の石油在庫の増加

• その他の要因：例えば、エコカー、省エネ製品の開発

Does Drug Interdiction Increase or DecreaseDrug-Related Crime?麻薬禁止法は麻薬からみ犯罪を減少できるか

• Drug interdiction impacts sellers rather than buyers.

麻薬禁止は買いてより売り手を制限する

• Demand is unchanged. Equilibrium price risesalthough quantity falls.

麻薬の需要は変わらない。そして、麻薬量の減少


麻薬の需要は変わらない。そして、麻薬量の減少にも関わらず均衡価格が上昇。

• Drug education impacts the buyers rather than sellers.麻薬使用の教育は買い手より売り手に効果がある

• Demand is shifted. Equilibrium price and quantityare lowered.

需要はシフトしたら、均衡価格と量は低下。

Price of Drugs Price of Drugs

Drug Interdiction 麻薬禁止 Drug Education 麻薬使用の教育

S2

S1S1

It is amazing how

Figure 9 Policies to Reduce the Use of Illegal Drugs不法麻薬使用減少の政策効果


Quantity of Drugs Quantity of Drugs

D2

D1

D1

The demand for illegal drugs is inelastic.Interdiction shifts the supply,

while education shifts the demand.

In each case, the change in price is the same. But in one market the price goes up.

And in the other it goes down.The changes in quantities (and TR) areremarkable.

It is amazing howuseful knowledgeof elasticities canbe!

Summary• Price elasticity of demand measures how much the quantity

demanded responds to changes in the price.

• Price elasticity of demand is calculated as the percentagechange in quantity demanded divided by the percentagechange in price.

– If a demand curve is elastic, total revenue falls when theprice rises.


price rises.

– If it is inelastic, total revenue rises as the price rises.

• 需要の価格弾力性はある商品の価格の変化により需要の量はどれだけ変化するかを測る尺度である。百分率で表示する。

• 普段は、商品の需要量は価格の変動に逆変動する。

Summary• The income elasticity of demand measures how much

the quantity demanded responds to changes inconsumers’ income.

• The cross-price elasticity of demand measures howmuch the quantity demanded of one good responds tothe price of another good.


the price of another good.

• The price elasticity of supply measures how much thequantity supplied responds to changes in the price.

供給の弾力性とは、価格の変化に供給量の変化率を量る尺度である。同じく百分率で表示する。

普段は、供給量と価格の変化は正の関係がある。

Summary• In most markets, supply is more elastic in the long

run than in the short run.

• 多くの場合は、供給量は短期より長期的に弾力的。

• The price elasticity of supply is calculated as thepercentage change in quantity supplied divided by the


percentage change in quantity supplied divided by thepercentage change in price.

• The tools of supply and demand can be applied inmany different types of markets.

• 供給と需要のツールは異なる市場の分析にも使われる。

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