Elasticity Of Demand From Micro Economics Perspecitive
Introduction
Price Elasticity of Demand
It is denoted as: Percentage Change in amount demanded/ Percentage Change in Price
Methods of Measuring Price Elasticity of Demand
The slope of the demand curve is different from its price elasticity. This fact is evidenced by measuring price elasticity on two demand curves of the same or different inclines.
However, there are
three exceptions to this and they are (1) When price and quantity are identical, it can be said by
looking at the inclines of the two intersecting demand curves which one is more or less elastic. This
is described in the nearby picture, where the incline of XQ curve shows that it is flatter and that of
the PR curve shows it to be steeper. Both intersect at point C so that they have identical price OT
and identical quantity OZ. If the demand curve is vertical, its price elasticity is zero and if the
demand curve is horizontal, its price elasticity is infinite.
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In this chapter we are going to discuss in detail the elasticity of demand. Elasticity of demand
in general denotes to price elasticity which is also called own price elasticity of demand.
Elasticity of demand also has the ideas of income, substitutes and cross elasticity of demand
and we are going to discuss all of them.
Price Elasticity of Demand
Price Elasticity of Demand is the grade of approachable demand of change in price. It is defined
by Prof. Lipsey as "Elasticity of demand may be defined as the ratio of the percentage change in
demand to the percentage change in price." It is also defined by Robinson as "The elasticity of
demand at any price....is the proportional change of amount purchased in response to a small
change in price, divided by the proportional change of price."
It is denoted as: Percentage Change in amount demanded/ Percentage Change in Price
Methods of Measuring Price Elasticity of Demand
There are four methods of approaching price elasticity of demand. They are discussed as follows.

The Percentage Method  It is determined by thee coefficient of price elasticity (Ep).
This coefficient determines the percentage change in the quantity demanded of an article from a specified
percentage price change. It is given by:
Ep = % change in q / % Change in p = (Δq / q) / (Δp / p) = (Δq / Δp) x p / q
Where q refers to quantity demanded, p to price and Δ to change. If Ep > 1, demand is elastic. If Ep < 1, demand is inelastic and if Ep = 1, demand is unity elastic. 
The Point Method  This method has been formulated by Prof. Marshall for measuring
elasticity at a point on the demand curve.
In this diagram, let us see how the point method works out. AB being straight line demand curve, if the price falls from PG(=OC) to FH(=OE), the quantity demanded is augmented from OG to OH. Elasticity at Point P on the AB demand curve as per the formulae is Ep = (Δq / Δp) x p / q, where Δq represents change in quantity demanded, Δp change in price level whereas p and q are prime price and quantity levels. From the pictorial representation. We derive the following. Δq = GH = QH, where, Δp = PQ.
P = PG and q = OG. Substituting these parameters in the specified elasticity formulae, we acquire: Ep = (QF / PQ) x (PG / OG). Besides, (QF / PQ) x (GB / PG). [ ∠PGB being right angles and PQF and PGB are similar Δb]. Hence, (GB / PG) x (PG / OG) = GB / OG.  The Arc Method  When elasticity is calculated between two points
on the same demand curve, it is termed as Arc elasticity. Prof. Baumol defines as "Arc elasticity
is a measure of the average responsiveness to price change exhibited by a demand curve over some
finite stretch of the curve." Any two points in the demand curve is an arc which measures elasticity
over a definite array of the price and quantity demanded. The formula for computing elasticity under
this method is as below.
Ep = [Δq / (q1+q2)^{∧}1/2] / [Δp / (p1+p2)^{∧}1/2] = (Δq / (q1+q2) ^{∧}1/2) x [(p1+p2)^{∧}1/2 / Δp) = (Δq / Δp) x [(p1+p2) / (q1+q2)]  The Total Outlay Method  Prof. Marshall has derived the total
outlay method of ascertaining the elasticity of demand. It is also termed as total revenue or total
expenditure method as a measure of elasticity. By contrasting the total outlay of a purchaser both
before and after the change in price, it can be known whether his demand for a good is elastic,
unity or less elastic. Total outlay is denoted as price proliferated with the quantity of an article
purchased that is denoted as below.
Total outlay = Price x Quantity Demanded
Demand is elastic, whilst with the drop in price, the sum of outlay augments and with the mount in price, the total outlay diminishes. The demand is unity when with the drop or mount in price, the total outlay stays invariable. Demand is less elastic with the consequence of drop in price drops total outlay and mount in price, mounts total outlay.
The slope of the demand curve is different from its price elasticity. This fact is evidenced by measuring price elasticity on two demand curves of the same or different inclines.
 Two straight line demand curve originating from
the same point
There are two straight line curves PQ and PR in this representation. PR is flatter than PQ and hence it appears that its price elasticity is higher than the other curve. But this is not actuality. If we further draw another line TS, passing through these curves and touching the vertical axis at point T, the elasticity at point U on the PQ curve as per the point formula is (QU/UP) = (OT/OP). Similarly, elasticity at point S on the PR curve is (RS/SP) = (OT/OP) and hence, (QU/UP) = (RS/SP) = (OT/OP) = 1.
Thus the elasticity is equal on both the points U and S of the two curves. We may conclude that if two linear demand curves originate from the vertical axis at the same point, such as P, they have exactly equal elasticity at every single price.  Two straight line demand curves originating from Different points which are
neither parallel nor intersecting 
This picture shows two demand curves PQ and XR. Of these, the curves XR is flatter and so it looks more elastic in price. But this is incorrect. To prove it, we shall draw another line from point T of the vertical axis which passes through these curves at points V and W respectively. Thus price elasticity of point V on the PQ curve is QV/VP = OT/TP and at point W on the XR curve is RW/WX = OT/TX. Since OT/TP > OT/TX, hence QV/VP > RW/WX.
It means that price elasticity of demand is less than 1 at point W on the demand curve XR and greater than 1 at point V on the PQ curve.  Two parallel straight line demand curve 
Two parallel straight line demand curves appear to have the same slope and hence the same price elasticity. Again this view is incorrect. To evidence this, let PQ and XR be two parallel straight line demand curves. We shall draw a line TU which passes through these straight lines at point Y and U correspondingly. As per the formula, elasticity of point Y on the curve PQ is QP/YP = OT/TP. Likewise, elasticity of point U on the XR curve is RU/UX = OT/TX. Since OT/TP > OT/TX, OT/TP > RU/UX. It means greater elasticity at point Y on the line PQ than at point U on the line XR.  Two points on a curved demand curve 
Let us consider the points IJ on a curved demand curve K. Elasticity at point J is QJ/JP and at point I, RI/IX. Since RI/IX is greater, elasticity at point I is greater than unity and at point J, it is less than unity. The above cases evidence that the price elasticity of demand cannot be computed by merely looking at the incline of a demand curve.
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Other topics under Consumption Theory:
 Cross Elasticity of Demand, Income Elasticity of Demand
 Demand and Law of Demand
 Exceptions to the Law of Demand
 The Concept of Customer's Surplus
 The Indifference Curve Theory  PART I
 The Indifference Curve Theory  PART II
 Price Earnings Line or Budget Line
 Revealed Preference Theory of Demand
 Superiority of Hicks' gauge of CS over Marshall's
 Superiority of Revealed Preference Theory
 Uses or Application of Indifference Curve Study