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In this question I asked about a way in order to find a specific transformation function $g(\cdot)$ in order to transform a random variable into another one.

Thanks to the answer to that question I could investigate a little more and understood that $g(\cdot)$ can be found using the CDFs of both distributions. So let's say that I have a random variable $X$ and I want to transform it into $Y$ knowing both $F_Y(y)$ and $F_X(x)$. The transformation $g(\cdot)$ that allows $Y = g(X)$ is the following:

$$ g(\cdot) = F_Y^{-1}(F_X(\cdot)) $$

How to handle discrete variables?

But I have a problem. The transformation $F_Y^{-1}(F_X(\cdot))$ cannot be done when I have discrete variables, or better when $Y$ is a discrete random variable.

It is obvious why. Being a discrete random variable $F_Y(y)$ will be a stair function, which is no a bijection! Inversion is not possible!

It seems that it is not possible to transform a generic random variable into a discrete one...

Question

My questions are:

  1. How to handle this, in the case I want $Y$ to be a discrete random variable?

  2. Is this method only for those cases where the destination random variable $Y$ is a continuos r.v.? Meaning that the method cannot transform a r.v. into a discrete one.

  3. Other than this approach, is there a method to get $g(\cdot)$ in order to achieve $Y=g(X)$ knowing everything about $X$ and $Y$ (moments, PMFs, CDFs and so on...)?

Thankyou

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1 Answer 1

ex: Convert $Y = ln(X)$.

First of all, we know that $X=e^Y$, so the domain of $Y$ is all real numbers. We also know the pdf of X is $F_X(x) = 1-e^x$. Then

$F_Y(y)=P(Y<y)=P(ln(x)\le Y)=P(x\le e^y)=F_X(e^y)$

So $F_Y(y)=F_X(e^y)$, then we differentiate to get $f_Y(y)=f_X(e^y)e^y$.

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Thankyou user65384 for your answer, but I am afraid to say that you did not get the point. I do not want to know how to get $F_Y(y)$, in my case I suppose I already have it. In your example you use $g(\cdot) = ln(\cdot)$ but in my question I pointed clear that $g$ must be $g(\cdot) =F_Y^{-1}(F_X(\cdot))$. In particular I need to consider the case where $F_Y(y)$ is a stair function that, so, cannot be inverted! –  Andry Mar 20 '13 at 8:27

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