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average: is exactly or approximately the quotient obtained by dividing the sum total of a set of figures by the number of figures: scored an ~ of 85 in a series of five tests.
mean: may be an average, OR it may represent value midway between two extremes: annual temperature ~ of 50 degree; the tranquil ~ between misery and ecstasy. 例1是平均数，例2是两极端的中间a value midway between two extremes. tranquil宁静的
median: applies to the value that represents the point at which there are as many instances above as there are below: the average of a group of persons earning 3,4,5,8,and 10 dollars a day is 6 dollars, whereas the ~ is 5 dollars. （词根MEDI表示中间middle，如mediate调停，medieval中世纪，intermediate中等中间，但mediocre平庸不是表示中间而是较差）
norm: denotes the computed or estimated average of performance of a significantly large group, class, or grade and implies a standard of reference: scores about the ~ for 5th grade arithmetic. 注意不要写成arithmatic
average平均数: 是一组数的和除以个数得到的商，mean中间值: 可表示平均数，或表示两极端的正中间，median中值: 在中值点以上和以下的个数相等，即一半比中值大，一半小，norm通例常规常态正态: 表示大量事例的平均值，能做参考标准
记忆方法：1）首字母AMMN想成Middle MAN中间人<==平均
2）平均的意思是两极端中间的点mean something that represents a middle point between extremes.
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