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The concept of consumer’s surplus in economics was first introduced by Alfred Marshall.

## Definition:

Marshall defined consumer surplus as:

“A consumer is generally willing to pay more for a given quantity of a good than what he actually pays at the price prevailing in the market”.

He further said about consumer’s surplus:

“The excess of the price which he (consumer) would be willing to pay rather than go without the thing over that which he actually does pay, is the economic measure of this surplus satisfaction”.

Anna Koutsoyiannis defined consumer surplus as:

Consumer’s surplus is equal to the difference between the amount of money that consumer actually pays to buy a certain quantity rather than go without it”.

## Assumptions:

According to Marshall, the concept of consumer surplus is based on following assumptions:

(i) Characteristic of consumer’s behavior.

(ii) Characteristic of market.

The characteristic of consumer’s behavior is that as he buys more and more of a particular commodity, the marginal utility of the successive units begins to decrease. A rational buyer continuous purchasing the commodity up to the unit which equates his marginal utility of the good to the price he pays for it.

The second phenomenon is that there is perfect competition among sellers and a single price prevails in the market for a particular commodity at a particular time. The buyer is able to get the first unit of the commodity at the same price as the second or pay any other unit thereafter.

## Calculation, Example, Table and Diagram:

The consumer’s surplus can be easily calculated by consumer’s demand curve for the commodity and the current market price which we assume a purchaser cannot change.

For example, you go to the market for the purchase of a shirt. You are mentally prepared to pay \$25 for the shirt which the seller has shown to you. He offers the shirt for \$10 only. You immediately purchase the shirt and say ‘thank you’. You were willing to pay \$25 for the shirt but you are delighted to get it for \$10 only.

Consumer’s surplus is the difference between the maximum amount which a consumer is willing to pay for the good and the price he actually pays for the good. In our example given above, the consumer’s surplus is \$15 (\$25 – \$10).

The concept of consumer’s surplus is now explain with the help of a table and a demand curve (diagram).

In this diagram 3.19, an individual is willing to purchase one shirt at the price of \$25, two shirts at the price of \$20, three shirts at the price of \$15 and four shirts at the price of \$10.

In case the price comes down to \$15 per shirt, the consumer purchases 3 shirts. By using this demand curve, we measure the surplus which a consumer gets from the purchase of shirts. The current market price of one shirt is \$10, which we have assumed the purchaser cannot change. The consumer was willing to pay \$25 per shirt but he actually pay \$10 only, the consumer’s surplus for the first shirt is \$15 = (25 – 10).

For the second shirt, it is \$10 = (20 – 10) and for the third, consumer’s surplus is \$5 = (15 – 10).

There is no surplus on the fourth unit as the market price for the shirt is the same what he would have paid for the shirt. The total consumer’s surplus from the purchase of four shirts is \$15 + \$10 + \$5 = \$30. It is the sum of surpluses received from each shirt. The shaded area in the graph shows the total consumer’s surplus.

## Criticism:

The Marshallian concept of consumer’s surplus has been severally criticized by modern economists Allen and Hicks. According to them, the concept is based on assumptions which are unwarranted. Utility, according to them, is a psychological feeling. It cannot be exactly measured in term of money.

In Marshallian analysis, the marginal utility of money is assumed to remain constant. The fact is that when a consumer spends money on goods, his income decreases and the marginal utility of money to him rises. Analysis ignores this basic fact.

Consumer’s surplus is said to be imaginary as it assumes that utilities derived from various goods are independent. In real life, this is not true. The fact is that utilities derived from various goods are independent.

## Calculation of Consumer’s Surplus with the help of Indifference Curves (Hicksian Method):

Professor J.R. Hicks, has explained the concept of consumer surplus with the help of indifference curve technique. According to Hicks when there is fall in the price of a commodity, it has two main effects:

First, the consumer can purchase more of the good whose price has fallen.

Secondly, he can purchase the same quantity of the good as he was buying before but with a lesser amount of money. He spares some money in the bargain. This is a form of rise in the real income of the consumer.

### Diagram:

The Hicksian method of measuring consumer’s surplus is now explained with the help of diagram below:

In figure 3.20 commodity X is measured on OX axis and money income of an individual on OY axis. We assume here that a consumer does not know the price of the commodity X and has OR quantity of money. The indifference curve IC1 represents various combinations of income and X of commodity X which yield the same level of satisfaction to the consumer.

The indifference curve IC1 originates from point R. It shows the stage when the consumer retains all of his income and zero units of commodity for a given level of the utility. The consumer moves down along the curve IC1. The consumer at point P buys OT amount of commodity X and has OE amount of money income. In other words, the consumer is ready to sacrifice RE amount of money for getting OT units of commodity X.

We now assume that the consumer is informed of the price of commodity X. The RL is the budget line. The budget line touches the indifference curve IC2 at point N which is the point of equilibrium. The consumer now has the OT commodity of X and OF amount of income. He gives up RF amount of money to buy OT units of commodity X. Previously he was ready to pay RE amount of income which is higher than the amount he pays now. We infer from this that RE – RF i.e., FE is the consumer surplus.

FE is the difference between the amount of income the consumer was willing to pay and what he actually pays.

The surplus has also shifted to the consumer on the higher level of satisfaction from IC1 to IC2.

## Importance and Use of Consumer’s Surplus:

The concept of consumer’s surplus has both theoretical as well as practical importance and use.

(i) Theoretical importance: The idea of consumer’s surplus reveals the benefits which we derive from our purchase of the commodity in the market.

For example, when we purchase salt, or a match box, we are willing to pay the amount much higher than their market value. For example, a consumer would be willing to pay \$10 for a match box rather than go without it but he actually pay Re one only on the purchase of a match box. Consumer’s surplus on the purchase of match box thus is \$ 9.0.

(ii) Practical importance: A monopolist can charge higher price for his product if the consumers are enjoying large consumers surplus on the use of his product.

(iii) The inhabitants of a country derive consumer’s surplus when they import commodities from abroad. They are usually prepared to pay more for than what they actually pay.

(iv) A finance minister imposes taxes of the commodities yielding consumer’s surplus.

(v) An entrepreneur before investing capital in a project evaluates the consumer’s surplus to be derived from it. If the benefits to the obtained are greater than the costs, the investment is undertaken.