Beta Quotient Distribution

One example of a beta quotient distribution determined by this script. Blue and orange lines show two independent beta distributions with parameters a=3, b=6 and a=12, b=7, respectively. Their expectation values are marked with vertical dashed lines. The green curve represents the PDF of the ratio of both random variables, whereas the red dash-dotted line is the respective CDF. The shaded regions are 90% credible intervals.

The Probability Density

According to [1] the probability density function (PDF) of the ratio of two random variables

which each follow the PDFs of beta distributions

and

respectively where the Beta function B(y,z) is

is given by

for 0 < w < 1 and

for w > 1.

The hypergeometric fuctions 2F1 take the form [2]

for w<1 and

for w>1.

The Cumulative Distribution

Using the fact that

and

one can calculate the integral of the PDF, the cumulative density function (CDF):

for w<1 and

for w>1.

Expectation Values

For the expectation value of a fraction x/y, where x and y follow beta distributions as above, one generally has:

with

and

where one uses the representation of the beta function with gamma functions as stated above together with their property

Hence, as result one obtains

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[1] Pham-Gia, T. "Distributions of the ratios of independent beta variables and applications." Communications in Statistics-Theory and Methods 29.12 (2000): 2693-2715.

[2] Luke, Yudell L., ed. Special functions and their approximations. Vol. 2. Academic press, 1969.