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In economics, demand is the quantities of a commodity or a service that people are willing AND able to buy at various prices, over a given period of time.^{[1]}
The relationship between price and quantity demanded is also known as demand curve. Preferences and choices, which underlie demand, can be represented as functions of cost, benefit, odds and other variables.
Determinants of (Factors affecting) demand Innumerable factors and circumstances could affect a buyer's willingness or ability to buy a good. Some of the common factors are:
Mathematically, the variable representing the price of the complementary good would have a negative coefficient in the demand function. For example, Q_{d} = a  P  P_{g} where Q is the quantity of automobiles demanded, P is the price of automobiles and P_{g} is the price of gasoline. The other main category of related goods are substitutes. Substitutes are goods that can be used in place of the primary good. The mathematical relationship between the price of the substitute and the demand for the good in question is positive. If the price of the substitute goes down the demand for the good in question goes down.
The demand equation is the mathematical expression of the relationship between the quantity of a good demanded and those factors that affect the willingness and ability of a consumer to buy the good. For example, Q_{d} = f(P; P_{rg}, Y) is a demand equation where Q_{d} is the quantity of a good demanded, P is the price of the good, P_{rg} is the price of a related good, and Y is income; the function on the right side of the equation is called the demand function. The semicolon in the list of arguments in the demand function means that the variables to the right are being held constant as one plots the demand curve in (quantity, price) space. A simple example of a demand equation is Q_{d} = 325  P  30P_{rg} + 1.4Y. Here 325 is the repository of all relevant nonspecified factors that affect demand for the product. P is the price of the good. The coefficient is negative in accordance with the law of demand. The related good may be either a complement or a substitute. If a complement, the coefficient of its price would be negative as in this example. If a substitute, the coefficient of its price would be positive. Income, Y, has a positive coefficient indicating that the good is a normal good. If the coefficient was negative the good in question would be an inferior good meaning that the demand for the good would fall as the consumer's income increased. Specifying values for the non price determinants, P_{rg} = 4.00 and Y = 50, results in the demand equation Q = 325  P  30(4) +1.4(50) or Q = 275  P. If income were to increase to 55 the new demand equation would be Q = 282  P. Graphically this change in a non price determinant of demand would be reflected in an outward shift of the demand function caused by a change in the x intercept.
PED is a measure of the sensitivity of the quantity variable, Q, to changes in the price variable, P. Elasticity answers the question of the percent by which the quantity demanded will change relative to (divided by) a given percentage change in the price. For infinitesimal changes the formula for calculating PED is the absolute value of (?Q/?P)×(P/Q).
=
The slope of a linear demand curve is constant. The elasticity of demand changes continuously as one moves down the demand curve because the ratio of price to quantity continuously falls. At the point the demand curve intersects the yaxis PED is infinitely elastic, because the variable Q appearing in the denominator of the elasticity formula is zero there. At the point the demand curve intersects the xaxis PED is zero, because the variable P appearing in the numerator of the elasticity formula is zero there.^{[2]} At one point on the demand curve PED is unitary elastic: PED equals one. Above the point of unitary elasticity is the elastic range of the demand curve (meaning that the elasticity is greater than one). Below is the inelastic range, in which the elasticity is less than one. The decline in elasticity as one moves down the curve is due to the falling P/Q ratio.
where a and c are parameters, and the constant price elasticity is c and .
In perfectly competitive markets the demand curve, the average revenue curve, and the marginal revenue curve all coincide and are horizontal at the marketgiven price.^{[3]} The demand curve is perfectly elastic and coincides with the average and marginal revenue curves. Economic actors are pricetakers. Perfectly competitive firms have zero market power; that is, they have no ability to affect the terms and conditions of exchange. A perfectly competitive firm's decisions are limited to whether to produce and if so, how much. In less than perfectly competitive markets the demand curve is negatively sloped and there is a separate marginal revenue curve. A firm in a less than perfectly competitive market is a pricesetter. The firm can decide how much to produce or what price to charge. In deciding one variable the firm is necessarily determining the other variable
In its standard form a linear demand equation is Q = a  bP. That is, quantity demanded is a function of price. The inverse demand equation, or price equation, treats price as a function g of quantity demanded: P = f(Q). To compute the inverse demand equation, simply solve for P from the demand equation.^{[4]} For example, if the demand equation is Q = 240  2P then the inverse demand equation would be P = 120  .5Q, the right side of which is the inverse demand function.^{[5]}
The inverse demand function is useful in deriving the total and marginal revenue functions. Total revenue equals price, P, times quantity, Q, or TR = P×Q. Multiply the inverse demand function by Q to derive the total revenue function: TR = (120  .5Q) × Q = 120Q  0.5Q². The marginal revenue function is the first derivative of the total revenue function; here MR = 120  Q. Note that the MR function has the same yintercept as the inverse demand function in this linear example; the xintercept of the MR function is onehalf the value of that of the demand function, and the slope of the MR function is twice that of the inverse demand function. This relationship holds true for all linear demand equations. The importance of being able to quickly calculate MR is that the profitmaximizing condition for firms regardless of market structure is to produce where marginal revenue equals marginal cost (MC). To derive MC the first derivative of the total cost function is taken. For example, assume cost, C, equals 420 + 60Q + Q^{2}. Then MC = 60 + 2Q. Equating MR to MC and solving for Q gives Q = 20. So 20 is the profit maximizing quantity: to find the profitmaximizing price simply plug the value of Q into the inverse demand equation and solve for P.
The demand curve facing a particular firm is called the residual demand curve. The residual demand curve is the market demand that is not met by other firms in the industry at a given price. The residual demand curve is the market demand curve D(p), minus the supply of other organizations, So(p): Dr(p) = D(p)  So(p )^{[6]}
Practically every introductory microeconomics text describes the demand curve facing a perfectly competitive firm as being flat or horizontal. A horizontal demand curve is perfectly elastic. If there are n identical firms in the market then the elasticity of demand PED facing any one firm is
where PED_{m} is the market elasticity of demand, PES is the elasticity of supply of each of the other firms, and (n 1) is the number of other firms. This formula suggests two things. The demand curve is not perfectly elastic and if there are a large number of firms in the industry the elasticity of demand for any individual firm will be extremely high and the demand curve facing the firm will be nearly flat.^{[6]}
For example, assume that there are 80 firms in the industry and that the demand elasticity for industry is 1.0 and the price elasticity of supply is 3. Then
That is the firm PED is 317 times as elastic as the market PED. If a firm raised its price "by one tenth of one percent demand would drop by nearly one third."^{[6]} if the firm raised its price by three tenths of one percent the quantity demanded would drop by nearly 100%. Three tenths of one percent marks the effective range of pricing power the firm has because any attempt to raise prices by a higher percentage will effectively reduce quantity demanded to zero.
Demand management in economics is the art or science of controlling economic or aggregate demand to avoid a recession. Such management is inspired by Keynesian macroeconomics, and Keynesian economics is sometimes referred to as demandside economics.
Negative demand: If the market response to a product is negative, it shows that people are not aware of the features of the service and the benefits offered. Under such circumstances, the marketing unit of a service firm has to understand the psyche of the potential buyers and find out the prime reason for the rejection of the service. For example: if passengers refuse a bus conductor's call to board the bus. The service firm has to come up with an appropriate strategy to remove the misunderstandings of the potential buyers. A strategy needs to be designed to transform the negative demand into a positive demand.
No demand: If people are unaware, have insufficient information about a service or due to the consumer's indifference this type of a demand situation could occur. The marketing unit of the firm should focus on promotional campaigns and communicating reasons for potential customers to use the firm's services. Service differentiation is one of the popular strategies used to compete in a no demand situation in the market.
Latent demand: At any given time it is impossible to have a set of services that offer total satisfaction to all the needs and wants of society. In the market there exists a gap between desirables and the availables. There is always a search on for better and newer offers to fill the gap between desirability and availability. Latent demand is a phenomenon of any economy at any given time, it should be looked upon as a business opportunity by service firms and they should orient themselves to identify and exploit such opportunities at the right time. For example, a passenger traveling in an ordinary bus dreams of traveling in a luxury bus. Therefore, latent demand is nothing but the gap between desirability and availability.
Seasonal demand:Some services do not have an all year round demand, they might be required only at a certain period of time. Seasons all over the world are very diverse. Seasonal demands create many problems to service organizations, such as: idling the capacity, fixed cost and excess expenditure on marketing and promotions. Strategies used by firms to overcome this hurdle are like  to nurture the service consumption habit of customers so as to make the demand unseasonal, or other than that firms recognize markets elsewhere in the world during the offseason period. Hence, this presents and opportunity to target different markets with the appropriate season in different parts of the world. For example, the need for Christmas cards comes around once a year. Or the, seasonal fruits in a country.
Demand patterns need to be studied in different segments of the market. Service organizations need to constantly study changing demands related to their service offerings over various time periods. They have to develop a system to chart these demand fluctuations, which helps them in predicting the demand cycles. Demands do fluctuate randomly, therefore, they should be followed on a daily, weekly or a monthly basis.
E. F. Schumacher challenges the prevailing economic assumption that fulfilling demand is the purpose of economic activity, offering a framework of what he calls "Buddhist economics" in which wise demands, fulfilling genuine human needs, are distinguished from unwise demands, arising from the five intellectual impairments recognized by Buddhism:^{[7]}
The cultivation and expansion of needs is the antithesis of wisdom. It is also the antithesis of freedom and peace. Every increase of needs tends to increase one's dependence on outside forces over which one cannot have control, and therefore increases existential fear. Only by a reduction of needs can one promote a genuine reduction in those tensions which are the ultimate causes of strife and war.^{[8]}
Demand reduction refers to efforts aimed at reducing the public desire for illegal and illicit drugs. The drug policy is in contrast to the reduction of drug supply, but the two policies are often implemented together.
Energy demand management, also known as demandside management (DSM) or demandside response (DSR), is the modification of consumer demand for energy through various methods such as financial incentives and behavioral change through education.