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I have two i.i.d. random variables $X$ and $Y$ and I want to generate a set of $n$ random samples from the distribution of the ratio between the two variables $Z=X/Y$.

The pdf of $Z$ is described by: $ f_Z(t) = \int_{-\infty}^\infty |u| ~ f_{X,Y}(tu,u)~\mathrm{d} u, $ and then its cdf: $ F_Z(t) = \int_{-\infty}^t f_Z(u)~\mathrm{d} u. $ Then, in theory I can generate random samples $z\sim Z$ by: $ z=F^{-1}_Z(r), $ where $r$ is a uniform random sample in $[0,1]$.

The problem is that obtaining $f_Z(t)$ is not straightforward, specially when $X$ and $Y$ are both Weibull distributions; neither it is to obtain $F_Z(t)$. Is there any way to obtain random samples from $Z$ by directly using random samples from the marginal distributions of $X$ and $Y$ ?

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    No, I meant samples from the marginal distributions of $X$ and $Y$. Actually $X$ and $Y$ are identical and independent variables.2012-04-11

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The question as clarified in a comment is trivial. Since $X$ and $Y$ are independent, samples from their marginal distributions can be combined to produce a sample from their joint distribution, and the quotient is a sample from $Z$.