<P> The graph of the demand curve uses the inverse demand function in which price is expressed as a function of quantity . The standard form of the demand equation can be converted to the inverse equation by solving for P or P = a / b − Q / b . </P> <P> More plainly, in the equation P = a − bQ, "a" is the intercept where price is zero (where the demand curve intercepts the Y - axis), "b" is the slope of the demand curve, "Q" is quantity and "P" is price . </P> <P> There is movement along a demand curve when a change in price causes the quantity demanded to change . It is important to distinguish between movement along a demand curve, and a shift in a demand curve . Movements along a demand curve happen only when the price of the good changes . When a non-price determinant of demand changes the curve shifts . These "other variables" are part of the demand function . They are "merely lumped into intercept term of a simple linear demand function ." Thus a change in a non-price determinant of demand is reflected in a change in the x-intercept causing the curve to shift along the x axis . </P> <P> The shift of a demand curve takes place when there is a change in any non-price determinant of demand, resulting in a new demand curve . Non-price determinants of demand are those things that will cause demand to change even if prices remain the same--in other words, the things whose changes might cause a consumer to buy more or less of a good even if the good's own price remained unchanged . Some of the more important factors are the prices of related goods (both substitutes and complements), income, population, and expectations . However, demand is the willingness and ability of a consumer to purchase a good under the prevailing circumstances; so, any circumstance that affects the consumer's willingness or ability to buy the good or service in question can be a non-price determinant of demand . As an example, weather could be a factor in the demand for beer at a baseball game . </P>

A movement on the demand curve will take place when there is
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