Mean
From Academic Kids

In statistics, mean has two related meanings:
 the average in ordinary English, which is more correctly called the arithmetic mean, to distinguish it from geometric mean or harmonic mean. The average is also called sample mean.
 the expected value of a random variable, which is also called the population mean.
As well as statistics, means are often used in geometry and analysis; a wide range of means have been developed for these purposes, which are not much used in statistics. See the "Other Means" section below for a list of means.
Sample mean is often used as an estimator of the central tendency such as the population mean. However, other estimators are also used. For example, the median is a more robust estimator of the central tendency than the sample mean.
For a realvalued random variable X, the mean is the expectation of X. If the expectation does not exist, then the random variable has no mean.
For a data set, the mean is just the sum of all the observations divided by the number of observations. Once we have chosen this method of describing the communality of a data set, we usually use the standard deviation to describe how the observations differ. The standard deviation is the square root of the average of squared deviations from the mean.
The mean is the unique value about which the sum of squared deviations is a minimum. If you calculate the sum of squared deviations from any other measure of central tendency, it will be larger than for the mean. This explains why the standard deviation and the mean are usually cited together in statistical reports.
An alternative measure of dispersion is the mean deviation, equivalent to the average absolute deviation from the mean. It is less sensitive to outliers, but less tractable when combining data sets.
The mean value of a function, <math>f(x)<math>, on an interval, <math>a
 <math>E(f(X))=\frac{\int_a^b f(x)\,dx}{ba}.<math>
Note that not every probability distribution has a defined mean or variance — see the Cauchy distribution for an example.
The following is a summary of some of the multiple methods for calculating the mean of a set of n numbers. See the table of mathematical symbols for explanations of the symbols used.
Contents 
Arithmetic mean
The arithmetic mean is the "standard" average, often simply called the "mean".
 <math> \bar{x} = {1 \over n} \sum_{i=1}^n{x_i} <math>
Do not confuse the mean with the median or mode! The mean is the arithmetic average of a set of values, or distribution; however, for skewed distributions, the mean is not the same as the middle value (median), or most likely (mode). For example, mean income is skewed upwards by a small number of people with very large incomes, so that the majority have an income lower than the mean. By contrast, the median income is the level at which half the population is below and half is above. The mode income is the most likely income, and favors the larger number of people with lower incomes. The median or mode are often more intuitive measures of such data.
That said, many skewed distributions are best described by their mean  such as the Exponential and Poisson distributions.
Geometric mean
The geometric mean is an average that is useful for sets of numbers that are interpreted according to their product and not their sum (as is the case with the arithmetic mean). For example rates of growth.
 <math> \bar{x} = \sqrt[n]{\prod_{i=1}^n{x_i}} <math>
Harmonic mean
The harmonic mean is an average which is useful for sets of numbers which are defined in relation to some unit, for example speed (distance per unit of time).
 <math> \bar{x} = \frac{n}{\sum_{i=1}^n \frac{1}{x_i}} <math>
Generalized mean
The generalized mean, also known as the power mean or Hölder mean, is an abstraction of the Arithmetic, Geometric and Harmonic Means.
 <math> \bar{x}(m) = \sqrt[m]{\frac{1}{n}\sum_{i=1}^n{x_i^m}} <math>
By choosing the appropriate value for the parameter m we can get the arithmetic mean (m = 1), the geometric mean (m > 0) or the harmonic mean (m = 1)
This could be generalised further as
 <math> \bar{x} = f^{1}\left({\frac{1}{n}\sum_{i=1}^n{f(x_i)}}\right) <math>
and again a suitable choice of an invertible f(x) will give the arithmetic mean with f(x)=x, the geometric mean with f(x)=log(x), and the harmonic mean with f(x)=1/x.
Weighted mean
The weighted mean is used, if one wants to combine average values from samples of the same population with different sample sizes:
 <math> \bar{x} = \frac{\sum_{i=1}^n{w_i \cdot x_i}}{\sum_{i=1}^n {w_i}} <math>
The weights <math>w_i<math> represent the bounds of the partial sample. In other applications they represent a measure for the reliability of the influence upon the mean by respective values.
Truncated mean
Sometimes a set of numbers (the data) might be contaminated by inaccurate outliers, i.e. values which are much too low or much too high. In this case one can use a truncated mean. It involves discarding given parts of the data at the top or the bottom end, typically an equal amount at each end, and then taking the arithmetic mean of the remaining data. The number of values removed is indicated as a percentage of total number of values.
Interquartile mean
The interquartile mean is a specific example of a truncated mean. It is simply the arithmetic mean after removing the lowest and the highest quarter of values.
 <math> \bar{x} = {2 \over n} \sum_{i=(n/4)+1}^{3n/4}{x_i} <math>
assuming the values have been ordered.
Other means
 Arithmeticgeometric mean
 Arithmeticharmonic mean
 Cesàro mean
 Chisini mean
 Geometricharmonic mean
 Heronian mean
 Identric mean
 Lehmer mean
 Quadratic mean
 root mean square
 Stolarsky mean
 weighted geometric mean
 weighted harmonic mean
 Rényi's entropy (a generalized fmean)
See also
External links
 Comparison between arithmetic and geometric mean of two numbers (http://www.sengpielaudio.com/calculatorgeommean.htm)de:Mittelwert