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A measure for the central tendency of a sample of observations. The term average is most often used for the arithmetic mean, but sometimes also for the median. For instance, suppose the yearly income of five people is $50,000, $80,000, $100,000, $120,000, and $650,000. The arithmetic mean is the sum of these values divided by their number, that is, $200,000. The median is obtained by ranking the values (as above) and taking the one in the middle, that is, $100,000. When the distribution is asymmetric, as it is often with income, the mean and the median are not the same, and it can be the case that most people earn less than the mean.

Another example is with legs. Most people (99.9%) have two legs. But some have had amputations or accidents, so the average number of legs in the population is less than two. Therefore 99.9% of the population have more legs than average.