Index of dispersion

Index of dispersion

In probability theory and statistics, the index of dispersion,[1] dispersion index, coefficient of dispersion, or variance-to-mean ratio (VMR), like the coefficient of variation, is a normalized measure of the dispersion of a probability distribution: it is a measure used to quantify whether a set of observed occurrences are clustered or dispersed compared to a standard statistical model.

It is defined as the ratio of the variance σ2 to the mean μ,

D = {\sigma^2 \over \mu }.

It is also known as the Fano factor, though this term is sometimes reserved for windowed data (the mean and variance are computed over a subpopulation), where the index of dispersion is the special case where the window is infinite. Windowing data is frequently done: the VMR is frequently computed over various intervals in time or small regions in space, which may be called "windows", and the resulting statistic called the Fano factor.

It is only defined when the mean μ is non-zero, and is generally only used for positive statistics, such as count data or time between events, or where the underlying distribution is assumed to be the exponential distribution or Poisson distribution.



In this context, the observed dataset may consist of the times of occurrence of predefined events, such as earthquakes in a given region over a given magnitude, or of the locations in geographical space of plants of a given species. Details of such occurrences are first converted into counts of the numbers of events or occurrences in each of a set of equal-sized time- or space-regions.

The above defines a dispersion index for counts.[2] A different definition applies for a dispersion index for intervals,[3] where the quantities treated are the lengths of the time-intervals between the events, and where the index is equivalent to the square of the coefficient of variation of the interval lengths. Common usage is that "index of dispersion" means the dispersion index for counts.


The Poisson distribution has equal variance and mean, giving it a VMR = 1. The geometric distribution and the negative binomial distribution have VMR > 1, while the binomial distribution has VMR < 1, and the constant random variable has VMR = 0. This yields the following table:

Distribution VMR
constant random variable VMR = 0 not dispersed
binomial distribution 0 < VMR < 1 under-dispersed
Poisson distribution VMR = 1
negative binomial distribution VMR > 1 over-dispersed

This can be considered analogous to the classification of conic sections by eccentricity; see Cumulants of particular probability distributions for details.

When the coefficient of dispersion is less than 1, a dataset is said to be "under-dispersed": this condition can relate to patterns of occurrence that are more regular than the randomness associated with a Poisson process. For instance, points spread uniformly in space or regular, periodic events will be under-dispersed.

If the index of dispersion is larger than 1, a dataset is said to be over-dispersed: this can correspond to the existence of clusters of occurrences. Clumped, concentrated data is over-dispersed.

In terms of the interval-counts, over-dispersion corresponds to there being more intervals with low counts and more intervals with high counts, compared to a Poisson distribution: in contrast, under-dispersion is characterised by there being more intervals having counts close to the mean count, compared to a Poisson distribution.

The relevance of the index of dispersion is that it has a value of one when the probability distribution of the number of occurrences in an interval is a Poisson distribution. Thus the measure can be used to assess whether observed data can be modeled using a Poisson process.

A sample-based estimate of the dispersion index can be used to construct a formal statistical hypothesis test for the adequacy of the model that a series of counts follow a Poisson distribution.[4][5]

The VMR is a good measure of the degree of randomness of a given phenomenon. This technique is also commonly used in currency management.


For randomly diffusing particles (Brownian motion), the distribution of the number of particle inside a given volume is poissonian, i.e. VMR=1. Therefore, to assess if a given spatial pattern (assuming you have a way to measure it) is due purely to diffusion or if some particle-particle interaction is involved : divide the space into patches, Quadrats or Sample Units (SU), count the number of individuals in each patch or SU, and compute the VMR. VMRs significantly higher than 1 denote a clustered distribution, where random walk is not enough to smother the attractive inter-particle potential.

See also

Similar ratios


  1. ^ Cox &Lewis (1966)
  2. ^ Cox & Lewis (1966), p72
  3. ^ Cox & Lewis (1966), p71
  4. ^ Cox & Lewis (1966), p158
  5. ^ Upton&Cook(2006), under index of dispersion


  • Cox, D. R., and Lewis, P.A.W. (1966) The Statistical Analysis of Series of Events Methuen, London
  • Upton, G., and Cook, I. (2006) Oxford Dictionary of Statistics (2nd edition). OUP. ISBN 978-0-19-954145-4

Wikimedia Foundation. 2010.

Игры ⚽ Поможем решить контрольную работу

Look at other dictionaries:

  • Dispersion — may refer to: In physics: The dependence of wave velocity on frequency or wavelength: Dispersion (optics), for light waves Dispersion (water waves) Acoustic dispersion, for sound waves Dispersion relation, the mathematical description of… …   Wikipedia

  • Dispersion-shifted fiber — (DSF) is a type of optical fiber made to optimize both low dispersion and low attenuation. Dispersion Shifted Fiber is a type of single mode optical fiber with a core clad index profile tailored to shift the zero dispersion wavelength from the… …   Wikipedia

  • dispersion — I noun allocation, decentralization, diffraction, diffusion, disjunction, dispensation, dispersal, dissemination, dissipation, distribution, divergence, division, emanation, parting, radiation, refraction, scattering, separation, spread II index… …   Law dictionary

  • Dispersion staining — Contents 1 Dispersion Staining 1.1 Becke Line Dispersion Staining 1.2 Oblique Illumination Dispersion Staining 1.3 Darkfield Illumination Dispersion Staining …   Wikipedia

  • Dispersion (optics) — This article is about dispersion of waves in optics. For other forms of dispersion, see Dispersion (disambiguation). In a prism, material dispersion (a wavelength dependent refractive index) causes different colors to refract at different angles …   Wikipedia

  • Dispersion relation — The refraction of a light in a prism is due to dispersion. In physics and electrical engineering, dispersion most often refers to frequency dependent effects in wave propagation. Note, however, that there are several other uses of the word… …   Wikipedia

  • dispersion — /di sperr zheuhn, sheuhn/, n. 1. Also, dispersal. an act, state, or instance of dispersing or of being dispersed. 2. Optics. a. the variation of the index of refraction of a transparent substance, as glass, with the wavelength of light, with the… …   Universalium

  • dispersion staining —    A technique for refractive index determination that employs central or annular stops placed in the objective back focal plane of a microscope. Using an annular stop with the substage iris closed, a fiber mounted in a high dispersion medium… …   Forensic science glossary

  • Index of optics articles — Optics is the branch of physics which involves the behavior and properties of light, including its interactions with matter and the construction of instruments that use or detect it.[1] Optics usually describes the behavior of visible,… …   Wikipedia

  • dispersion — dis•per•sion [[t]dɪˈspɜr ʒən, ʃən[/t]] n. 1) Also, dispersal an act or instance of dispersing or a state of being dispersed. 2) opt a) the variation of the index of refraction of a transparent substance, as glass, with the wavelength of light b)… …   From formal English to slang

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”