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Moment (mathematics)
It has been suggested that Central moment be merged into this article. (Discuss) Proposed since July 2014. 
In mathematics, a moment is a specific quantitative measure, used in both mechanics and statistics, of the shape of a set of points. If the points represent mass, then the zeroth moment is the total mass, the first moment divided by the total mass is the center of mass, and the second moment is the rotational inertia. If the points represent probability density, then the zeroth moment is the total probability (i.e. one), the first moment is the mean, the second moment is the variance, and the third moment is the skewness. The mathematical concept is closely related to the concept of moment in physics.
For a bounded distribution of mass or probability, the collection of all the moments (of all orders, from 0 to ∞) uniquely determines the distribution.
Contents
Significance of the moments
The nth moment of a realvalued continuous function f(x) of a real variable about a value c is
 <math>\mu_n=\int_{\infty}^\infty (x  c)^n\,f(x)\,dx.</math>
It is possible to define moments for random variables in a more general fashion than moments for real values—see moments in metric spaces. The moment of a function, without further explanation, usually refers to the above expression with c = 0.
For the second and higher moments, the central moments (moments about the mean, with c being the mean) are usually used rather than the moments about zero, because they provide clearer information about the distribution's shape.
Other moments may also be defined. For example, the nth inverse moment about zero is <math>\operatorname{E}\left[X^{n}\right]</math> and the nth logarithmic moment about zero is <math>\operatorname{E}\left[\ln^n(X)\right].</math>
The nth moment about zero of a probability density function f(x) is the expected value of X^{n} and is called a raw moment or crude moment.^{[1]} The moments about its mean μ are called central moments; these describe the shape of the function, independently of translation.
If f is a probability density function, then the value of the integral above is called the nth moment of the probability distribution. More generally, if F is a cumulative probability distribution function of any probability distribution, which may not have a density function, then the nth moment of the probability distribution is given by the Riemann–Stieltjes integral
 <math>\mu'_n = \operatorname{E} \left [ X^n \right ] =\int_{\infty}^\infty x^n\,dF(x)\,</math>
where X is a random variable that has this cumulative distribution F, and E is the expectation operator or mean.
When
 <math>\operatorname{E}\left [\left X^n \right  \right ] = \int_{\infty}^\infty x^n\,dF(x) = \infty,</math>
then the moment is said not to exist. If the nth moment about any point exists, so does the (n − 1)th moment (and thus, all lowerorder moments) about every point.
The zeroth moment of any probability density function is 1, since the area under any probability density function must be equal to one.
Moment number  Raw moment  Central moment  Standardised moment  Raw cumulant  Standardised cumulant 

1  mean  0  0  mean  N/A 
2  –  variance  1  variance  1 
3  –  –  skewness  –  skewness 
4  –  –  historical kurtosis (or flatness)  –  modern kurtosis (i.e. excess kurtosis) 
5  –  –  hyperskewness  –  – 
6  –  –  hyperflatness  –  – 
7+  –  –    –  – 
Mean
The first raw moment is the mean.
Variance
The second central moment is the variance. Its positive square root is the standard deviation σ.
Normalised moments
The normalised nth central moment or standardized moment is the nth central moment divided by σ^{n}; the normalised nth central moment of
 <math>x = \frac{\operatorname{E} \left [(x  \mu)^n \right ]}{\sigma^n}.</math>
These normalised central moments are dimensionless quantities, which represent the distribution independently of any linear change of scale.
For an electric signal, the first moment is its DC level, and the 2nd moment is proportional to its average power.^{[2]}^{[3]}
Skewness
The third central moment is a measure of the lopsidedness of the distribution; any symmetric distribution will have a third central moment, if defined, of zero. The normalised third central moment is called the skewness, often γ. A distribution that is skewed to the left (the tail of the distribution is longer on the left) will have a negative skewness. A distribution that is skewed to the right (the tail of the distribution is longer on the right), will have a positive skewness.
For distributions that are not too different from the normal distribution, the median will be somewhere near μ − γσ/6; the mode about μ − γσ/2.
Kurtosis
The fourth central moment is a measure of whether the distribution is tall and skinny or short and squat, compared to the normal distribution of the same variance. Since it is the expectation of a fourth power, the fourth central moment, where defined, is always positive; and except for a point distribution, it is always strictly positive. The fourth central moment of a normal distribution is 3σ^{4}.
The kurtosis κ is defined to be the normalised fourth central moment minus 3 (Equivalently, as in the next section, it is the fourth cumulant divided by the square of the variance). Some authorities do not subtract three, but it is usually more convenient to have the normal distribution at the origin of coordinates.^{[4]}^{[5]} If a distribution has a peak at the mean and long tails, the fourth moment will be high and the kurtosis positive (leptokurtic); conversely, bounded distributions tend to have low kurtosis (platykurtic).
The kurtosis can be positive without limit, but κ must be greater than or equal to γ^{2} − 2; equality only holds for binary distributions. For unbounded skew distributions not too far from normal, κ tends to be somewhere in the area of γ^{2} and 2γ^{2}.
The inequality can be proven by considering
 <math>\operatorname{E} \left [(T^2  aT  1)^2 \right ]</math>
where T = (X − μ)/σ. This is the expectation of a square, so it is nonnegative for all a; however it is also a quadratic polynomial in a. Its discriminant must be nonpositive, which gives the required relationship.
Mixed moments
Mixed moments are moments involving multiple variables.
Some examples are covariance, coskewness and cokurtosis. While there is a unique covariance, there are multiple coskewnesses and cokurtoses.
Higher moments
Highorder moments are moments beyond 4thorder moments. As with variance, skewness, and kurtosis, these are higherorder statistics, involving nonlinear combinations of the data, and can be used for description or estimation of further shape parameters. The higher the moment, the harder it is to estimate, in the sense that larger samples are required in order to obtain estimates of similar quality. This is due to the excess degrees of freedom consumed by the higher orders. Further, they can be subtle to interpret, often being most easily understood in terms of lower order moments – compare the higher derivatives of jerk and jounce in physics. For example, just as the 4thorder moment (kurtosis) can be interpreted as "relative importance of tails versus shoulders in causing dispersion" (for a given dispersion, high kurtosis corresponds to heavy tails, while low kurtosis corresponds to heavy shoulders), the 5thorder moment can be interpreted as measuring "relative importance of tails versus center (mode, shoulders) in causing skew" (for a given skew, high 5th moment corresponds to heavy tail and little movement of mode, while low 5th moment corresponds to more change in shoulders).
Cumulants
The first moment and the second and third unnormalized central moments are additive in the sense that if X and Y are independent random variables then
 <math>\begin{align}
\mu_1(X+Y) &= \mu_1(X)+\mu_1(Y) \\ \operatorname{Var}(X+Y) &=\operatorname{Var}(X) + \operatorname{Var}(Y) \\ \mu_3(X+Y) &=\mu_3(X)+\mu_3(Y) \end{align}</math>
(These can also hold for variables that satisfy weaker conditions than independence. The first always holds; if the second holds, the variables are called uncorrelated).
In fact, these are the first three cumulants and all cumulants share this additivity property.
Sample moments
For all k, the kth raw moment of a population can be estimated using the kth raw sample moment
 <math>\frac{1}{n}\sum_{i = 1}^{n} X^k_i</math>
applied to a sample X_{1}, ..., X_{n} drawn from the population.
It can be shown that the expected value of the raw sample moment is equal to the kth raw moment of the population, if that moment exists, for any sample size n. It is thus an unbiased estimator. This contrasts with the situation for central moments, whose computation uses up a degree of freedom by using the sample mean. So for example an unbiased estimate of the population variance (the second central moment) is given by
 <math>\frac{1}{n1}\sum_{i = 1}^{n} (X_i\bar X)^2</math>
in which the previous denominator n has been replaced by the degrees of freedom n − 1, and in which <math>\bar X</math> refers to the sample mean. This estimate of the population moment is greater than the unadjusted observed sample moment by a factor of <math>\tfrac{n}{n1},</math> and it is referred to as the "adjusted sample variance" or sometimes simply the "sample variance".
Problem of moments
The problem of moments seeks characterizations of sequences { μ′_{n} : n = 1, 2, 3, ... } that are sequences of moments of some function f.
Partial moments
Partial moments are sometimes referred to as "onesided moments." The nth order lower and upper partial moments with respect to a reference point r may be expressed as
 <math>\mu_n^(r)=\int_{\infty}^r (r  x)^n\,f(x)\,dx,</math>
 <math>\mu_n^+(r)=\int_r^\infty (x  r)^n\,f(x)\,dx.</math>
Partial moments are normalized by being raised to the power 1/n. The upside potential ratio may be expressed as a ratio of a firstorder upper partial moment to a normalized secondorder lower partial moment. They have been used in the definition of some financial metrics, such as the Sortino ratio, as they focus purely on upside or downside.
Central moments in metric spaces
Let (M, d) be a metric space, and let B(M) be the Borel σalgebra on M, the σalgebra generated by the dopen subsets of M. (For technical reasons, it is also convenient to assume that M is a separable space with respect to the metric d.) Let 1 ≤ p ≤ ∞.
The pth central moment of a measure μ on the measurable space (M, B(M)) about a given point x_{0} ∈ M is defined to be
 <math>\int_{M} d(x, x_{0})^{p} \, \mathrm{d} \mu (x).</math>
μ is said to have finite pth central moment if the pth central moment of μ about x_{0} is finite for some x_{0} ∈ M.
This terminology for measures carries over to random variables in the usual way: if (Ω, Σ, P) is a probability space and X : Ω → M is a random variable, then the pth central moment of X about x_{0} ∈ M is defined to be
 <math>\int_{M} d (x, x_{0})^{p} \, \mathrm{d} \left( X_{*} (\mathbf{P}) \right) (x) \equiv \int_{\Omega} d (X(\omega), x_{0})^{p} \, \mathrm{d} \mathbf{P} (\omega),</math>
and X has finite pth central moment if the pth central moment of X about x_{0} is finite for some x_{0} ∈ M.
See also
 Factorial moment
 Generalised mean
 Hamburger moment problem
 Hausdorff moment problem
 Image moment
 Lmoment
 Method of moments (probability theory)
 Method of moments (statistics)
 Momentgenerating function
 Moment measure
 Second moment method
 Standardised moment
 Stieltjes moment problem
 Taylor expansions for the moments of functions of random variables
References
 ^ http://mathworld.wolfram.com/RawMoment.html Raw Moments at Mathworld
 ^ Clive Maxfield; John Bird; Tim Williams; Walt Kester; Dan Bensky (2011). Electrical Engineering: Know It All. Newnes. p. 884. ISBN 9780080949666.
 ^ Ha H. Nguyen; Ed Shwedyk (2009). A First Course in Digital Communications. Cambridge University Press. p. 87. ISBN 9780521876131.
 ^ Casella, George; Berger, Roger L. (2002). Statistical Inference (2 ed.). Pacific Grove: Duxbury. ISBN 0534243126.
 ^ Ballanda, Kevin P.; MacGillivray, H. L. (1988). "Kurtosis: A Critical Review". The American Statistician (American Statistical Association) 42 (2): 111–119. JSTOR 2684482. doi:10.2307/2684482.
Further reading
 Spanos, Aris (1999). Probability Theory and Statistical Inference. New York: Cambridge University Press. pp. 109–130. ISBN 0521424089.
External links
 Hazewinkel, Michiel, ed. (2001), "Moment", Encyclopedia of Mathematics, Springer, ISBN 9781556080104
 Moments at Mathworld
 Higher Moments
