Mean

The mean for a population characteristic is the average for that characteristic, for example, the mean height of a population is the average height of the population. We can think about the mean as a parameter; in that case, we mean the true mean, or the true average height of the population, accounting for every member of that population. But we can also think of the mean as a statistic: a quantity we estimate.

The basic formula for the mean (the statistic) is:

Taken from http://en.wikipedia.org/wiki/Mean
Source: Wikipedia [disclaimer]

where n is the size of the simple random sample used to create the statistic, and xi is one observation, or one individual's (individual i's) height in the example given above. The mean of an entire population (the parameter mean) uses the same formula; the only difference is that the height of every individual is used (and n then becomes N, the size of the population).

Calculating means becomes more difficult in cases where data are collected through a method other than a simple random sample. For information on weighted means, please see the Wikipedia article (26 December 2006) [disclaimer].