A distribution often used as a prior distribution for a proportion. The probability density function, for a random variable X having a beta distribution iswhere α and β are positive parameters, and B is the beta function. The name appears in a 1911 publication by Gini. The distribution has meanand varianceIf both α>1 and β>1, the distribution has mode atIf both α<1 and β<1 then the distribution is U-shaped, whereas, if just one of α and β is<1, then the distribution is J-shaped.
If Y1, Y2,…, Yk are independent random variables, with Yj having a chi-squared distribution with νj degrees of freedom, then the ratiohas a beta distribution with and .