Editing Beta distribution (section)

===Relationships between statistical measures===

====Mean, mode and median relationship====
If 1 < ''α'' < ''β'' then mode ≤ median ≤ mean.<ref name=Kerman2011>{{cite arXiv | eprint=1111.0433 | last1=Kerman | first1=Jouni | title=A closed-form approximation for the median of the beta distribution | date=2011 | class=math.ST }}</ref> Expressing the mode (only for ''α'', ''β'' > 1), and the mean in terms of ''α'' and ''β'':

: <math> \frac{ \alpha - 1 }{ \alpha + \beta - 2 } \le \text{median} \le \frac{ \alpha }{ \alpha + \beta } ,</math>

If 1 < ''β'' < ''α'' then the order of the inequalities are reversed. For ''α'', ''β'' > 1 the absolute distance between the mean and the median is less than 5% of the distance between the maximum and minimum values of ''x''. On the other hand, the absolute distance between the mean and the mode can reach 50% of the distance between the maximum and minimum values of ''x'', for the ([[Pathological (mathematics)|pathological]]) case of ''α'' = 1 and ''β'' = 1, for which values the beta distribution approaches the uniform distribution and the [[information entropy|differential entropy]] approaches its [[Maxima and minima|maximum]] value, and hence maximum "disorder".

For example, for ''α'' = 1.0001 and ''β'' = 1.00000001:
* mode   = 0.9999;   PDF(mode) = 1.00010
* mean   = 0.500025; PDF(mean) = 1.00003
* median = 0.500035; PDF(median) = 1.00003
* mean − mode   = −0.499875
* mean − median = −9.65538 × 10<sup>−6</sup>

where PDF stands for the value of the [[probability density function]].

[[File:Mean Median Difference - Beta Distribution for alpha and beta from 1 to 5 - J. Rodal.jpg|325px]]
[[File:Mean Mode Difference - Beta Distribution for alpha and beta from 1 to 5 - J. Rodal.jpg|325px]]

====Mean, geometric mean and harmonic mean relationship====
[[File:Mean, Median, Geometric Mean and Harmonic Mean for Beta distribution with alpha = beta from 0 to 5 - J. Rodal.png|thumb|:Mean, median, geometric mean and harmonic mean for beta distribution with 0 < ''α'' = ''β'' < 5]]

It is known from the [[inequality of arithmetic and geometric means]] that the geometric mean is lower than the mean.  Similarly, the harmonic mean is lower than the geometric mean.  The accompanying plot shows that for ''α'' = ''β'', both the mean and the median are exactly equal to 1/2, regardless of the value of ''α'' = ''β'', and the mode is also equal to 1/2 for ''α'' = ''β'' > 1, however the geometric and harmonic means are lower than 1/2 and they only approach this value asymptotically as ''α'' = ''β'' → ∞.

====Kurtosis bounded by the square of the skewness====
[[File:(alpha and beta) Parameter estimates vs. excess Kurtosis and (squared) Skewness Beta distribution - J. Rodal.png|thumb|left|Beta distribution ''α'' and ''β'' parameters vs. excess kurtosis and squared skewness]]

As remarked by [[William Feller|Feller]],<ref name=Feller /> in the [[Pearson distribution|Pearson system]] the beta probability density appears as [[Pearson distribution|type I]] (any difference between the beta distribution and Pearson's type I distribution is only superficial and it makes no difference for the following discussion regarding the relationship between kurtosis and skewness). [[Karl Pearson]] showed, in Plate 1 of his paper <ref name=Pearson>{{cite journal
 | last = Pearson
 | first = Karl
 | author-link = Karl Pearson
 | year = 1916
 | title = Mathematical contributions to the theory of evolution, XIX: Second supplement to a memoir on skew variation
 | journal = Philosophical Transactions of the Royal Society A
 | volume = 216
 | issue =538–548
 | pages = 429&ndash;457
 | doi = 10.1098/rsta.1916.0009
 | jstor=91092|bibcode = 1916RSPTA.216..429P | doi-access = free
 }}</ref>  published in 1916,  a graph with the [[kurtosis]] as the vertical axis ([[ordinate]]) and the square of the [[skewness]] as the horizontal axis ([[abscissa]]), in which a number of distributions were displayed.<ref name=Egon>{{cite journal|last=Pearson|first=Egon S.|title=Some historical reflections traced through the development of the use of frequency curves|journal=THEMIS Statistical Analysis Research Program, Technical Report 38|date=July 1969|volume=Office of Naval Research, Contract N000014-68-A-0515|issue=Project NR 042–260|url=http://www.smu.edu/Dedman/Academics/Departments/Statistics/Research/TechnicalReports}}</ref>  The region occupied by the beta distribution is bounded by the following two [[Line (geometry)|lines]] in the (skewness<sup>2</sup>,kurtosis) [[Cartesian coordinate system|plane]], or the (skewness<sup>2</sup>,excess kurtosis) [[Cartesian coordinate system|plane]]:

:<math>(\text{skewness})^2+1< \text{kurtosis}< \frac{3}{2} (\text{skewness})^2 + 3</math>

or, equivalently,

:<math>(\text{skewness})^2-2< \text{excess kurtosis}< \frac{3}{2} (\text{skewness})^2</math>

At a time when there were no powerful digital computers, [[Karl Pearson]] accurately computed further boundaries,<ref name="Hahn and Shapiro">{{cite book|last1=Hahn|first1=Gerald J.|last2=Shapiro|first2=S.|title=Statistical Models in Engineering (Wiley Classics Library)|year=1994|publisher=Wiley-Interscience|isbn=978-0471040651}}</ref><ref name=Pearson /> for example, separating the "U-shaped" from the "J-shaped" distributions. The lower boundary line (excess kurtosis + 2 − skewness<sup>2</sup> = 0) is produced by skewed "U-shaped" beta distributions with both values of shape parameters ''α'' and ''β'' close to zero.  The upper boundary line (excess kurtosis − (3/2) skewness<sup>2</sup> = 0) is produced by extremely skewed distributions with very large values of one of the parameters and very small values of the other parameter.  [[Karl Pearson]] showed<ref name=Pearson/> that this upper boundary line (excess kurtosis − (3/2) skewness<sup>2</sup> = 0) is also the intersection with Pearson's distribution III, which has unlimited support in one direction (towards positive infinity), and can be bell-shaped or J-shaped. His son, [[Egon Pearson]], showed<ref name=Egon/> that the region (in the kurtosis/squared-skewness plane) occupied by the beta distribution (equivalently, Pearson's distribution I) as it approaches this boundary (excess kurtosis − (3/2) skewness<sup>2</sup> = 0) is shared with the [[noncentral chi-squared distribution]].  Karl Pearson<ref name=Pearson1895>{{cite journal | last = Pearson | first = Karl | author-link = Karl Pearson | year = 1895 | title = Contributions to the mathematical theory of evolution, II: Skew variation in homogeneous material | journal = Philosophical Transactions of the Royal Society | volume = 186 | pages = 343&ndash;414 | doi = 10.1098/rsta.1895.0010 | jstor=90649 | bibcode=1895RSPTA.186..343P| doi-access = free }}</ref> (Pearson 1895, pp.&nbsp;357, 360, 373–376) also showed that the [[gamma distribution]] is a Pearson type III distribution. Hence this boundary line for Pearson's type III distribution is known as the gamma line. (This can be shown from the fact that the excess kurtosis of the gamma distribution is 6/''k'' and the square of the skewness is 4/''k'', hence (excess kurtosis − (3/2) skewness<sup>2</sup> = 0) is identically satisfied by the gamma distribution regardless of the value of the parameter "k"). Pearson later noted that the [[chi-squared distribution]] is a special case of Pearson's type III and also shares this boundary line (as it is apparent from the fact that for the [[chi-squared distribution]] the excess kurtosis is 12/''k'' and the square of the skewness is 8/''k'', hence (excess kurtosis − (3/2) skewness<sup>2</sup> = 0) is identically satisfied regardless of the value of the parameter "k"). This is to be expected, since the chi-squared distribution ''X'' ~ χ<sup>2</sup>(''k'') is a special case of the gamma distribution, with parametrization X ~ Γ(k/2, 1/2) where k is a positive integer that specifies the "number of degrees of freedom" of the chi-squared distribution.

An example of a beta distribution near the upper boundary (excess kurtosis − (3/2) skewness<sup>2</sup> = 0) is given by α = 0.1, β = 1000, for which the ratio (excess kurtosis)/(skewness<sup>2</sup>) = 1.49835 approaches the upper limit of 1.5 from below. An example of a beta distribution near the lower boundary (excess kurtosis + 2 − skewness<sup>2</sup> = 0) is given by α= 0.0001, β = 0.1, for which values the expression (excess kurtosis + 2)/(skewness<sup>2</sup>) = 1.01621 approaches the lower limit of 1 from above. In the infinitesimal limit for both ''α'' and ''β'' approaching zero symmetrically, the excess kurtosis reaches its minimum value at −2.  This minimum value occurs at the point at which the lower boundary line intersects the vertical axis ([[ordinate]]). (However, in Pearson's original chart, the ordinate is kurtosis, instead of excess kurtosis, and it increases downwards rather than upwards).

Values for the skewness and excess kurtosis below the lower boundary (excess kurtosis + 2 − skewness<sup>2</sup> = 0) cannot occur for any distribution, and hence [[Karl Pearson]] appropriately called the region below this boundary the "impossible region". The boundary for this "impossible region" is determined by (symmetric or skewed) bimodal U-shaped distributions for which the parameters ''α'' and ''β'' approach zero and hence all the probability density is concentrated at the ends: ''x'' = 0, 1 with practically nothing in between them. Since for ''α'' ≈ ''β'' ≈ 0 the probability density is concentrated at the two ends ''x'' = 0 and ''x'' = 1, this "impossible boundary" is determined by a [[Bernoulli distribution]], where the two only possible outcomes occur with respective probabilities ''p'' and ''q'' = 1&nbsp;−&nbsp;''p''. For cases approaching this limit boundary with symmetry ''α'' = ''β'', skewness ≈ 0, excess kurtosis ≈ −2 (this is the lowest excess kurtosis possible for any distribution), and the probabilities are ''p'' ≈ ''q'' ≈ 1/2.  For cases approaching this limit boundary with skewness, excess kurtosis ≈ −2 + skewness<sup>2</sup>, and the probability density is concentrated more at one end than the other end (with practically nothing in between), with probabilities <math>p = \tfrac{\beta}{\alpha + \beta}</math> at the left end ''x'' = 0 and <math>q = 1-p = \tfrac{\alpha}{\alpha + \beta}</math> at the right end ''x'' = 1.