什麼是正常財和劣等財

Normal good

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In economics, normal goods are any goods for which demand increases when income increases, i.e. with a positive income elasticity of demand. The term does not refer to the quality of the good.
Depending on the indifference curves, the amount of a good bought can either increase, decrease, or stay the same when income increases. In the diagram below, good Y is a normal good since the amount purchased increases from Y1 to Y2 as the budget constraint shifts from BC1 to the higher income BC2. Good X is an inferior good since the amount bought decreases from X1 to X2 as income increases.

圖片參考：http://upload.wikimedia.org/wikipedia/en/5/5a/Inferior_good.png

In consumer theory, an inferior good is a good that decreases in demand when the consumer's income rises, unlike normal goods, for which the opposite is observed. Inferiority, in this sense, is an observable fact rather than a statement about the quality of the good. As a rule, surfeit is easily achieved with such goods, and as more costly substitutes that offer more pleasure or at least variety become available, the use of the inferior goods diminishes.
Inter-city bus service is an example of an inferior good. This form of transportation is cheaper than air or rail travel, but is more time-consuming. When money is constricted, in comparison to time, traveling by bus becomes more palatable, but when money is more abundant than time, thus, worth more of it, more rapid transport is preferred.
Inexpensive foods like potatoes, hamburger, mass-market beer, frozen dinners, and canned goods are additional examples of inferior goods. As a person's income rises one tends purchases to more expensive foods. Likewise, objects heavily used by poor people (televisions) and related services (television repair) for which richer people have alternatives exemplify inferior goods. As a rule, used and obsolete goods (but not antiques!) marketed to persons of low income as closeouts are inferior goods at the time even if they had earlier been normal goods or even luxury goods.
Depending on consumer or market indifference curves, the amount of a good bought can either increase, decrease, or stay the same when income increases. In the diagram below, good Y is a normal good since the amount purchased increases from Y1 to Y2 as the budget constraint shifts from BC1 to the higher income BC2. Good X is an inferior good since the amount bought decreases from X1 to X2 as income increases.
Example of a normal good and inferior.

圖片參考：http://upload.wikimedia.org/wikipedia/en/5/5a/Inferior_good.png

面對Inferior good的問題時, we can call to the consumer's council and discuss/complain about these.

正常財: normal / superior good is a good whose quantity demand increases as real income rises.
劣等財: inferior good is a good whose quantity demand falls as real income rises.

For example, when your real income rises, you may order more abalone in place of raw bread. When you are extremely poor (real income is low) you may choose to eat more bread but not abalone at all. In this case, abalone is normal / superior good while bread is inferior good.