= How do I produce random variables which follow a negatively skewed distribution? =
Most distributions such as the exponential and log-Normal distributions are positively skewed with the mode of the distribution occurring for lower values.
The Weibull distribution is negatively skewed and may be generated [[http://www.taygeta.com/random/weibull.xml|using random variables which are uniform on the interval (0,1).]]
The below produces an open ended negatively skewed weibull distribution with parameters, 2 and 20. It has a median of
$$2^text{-0.05}ln(2)^text{0.05}$$
= 0.95.
which is of form
$$A^text{-1/B}ln(2)^text{1/B}$$
where A and B are the two parameters of the Weibull distribution.
(See [[http://www.weibull.com/AccelTestWeb/weibull_distribution.htm|here for formulae).]]
{{{
compute alpha=2.
compute beta=20.
compute rvw=((-1/alpha)*(ln(1-rv.uniform(0,1))))**(1/beta).
compute med=((alpha)**(-1/beta))*(ln(2)**(1/beta)).
exe.
}}}