# How to find the deterministic function representation of a random variable?

For any random variable $X$ whose density is $\mathbb{P}(X=x)=p(x;\theta),$ where $\theta$ is a parameter, its deterministic function representation is $X=f(\theta, \omega)$ where $\omega$ is a random variable whose distribution is independent of $\theta$.

Question: what is the condition on $p(x; \theta)$ so that its deterministic function representation exists?

For example, if $X\sim N(\mu,\sigma^2)$, then $X=\mu+\sigma \omega$ where $\omega$ is a standard normal random variable.

If $X \sim \operatorname{Bernoulli}(q)$, then $X=1(\operatorname{logit}^{-1}(q)>\omega)$ where $\omega$ is a logistic random variable.

I don't know if there is a deterministic function representation for a Poisson random variable.

• If $\omega$ is uniform$(0, 1)$ then I believe such a function $f$ always exists in accordance with the probability transform. en.wikipedia.org/wiki/Probability_integral_transform – dsaxton Dec 8 '15 at 20:06
• Yes but it may depend on the parameter $\theta$. The question is whether one can have a separation. – hchen Dec 8 '15 at 20:09
• In your post you required only that the distribution of $\omega$ not depend on $\theta$. Obviously $f$ should depend on $\theta$. – dsaxton Dec 8 '15 at 20:11
• Ah, I see your argument. Yes I believe this is true. – hchen Dec 8 '15 at 20:16
• It's a nice result because it suggests you can imagine there being a single primitive source of randomness, and all random variables as coming about by "stretching" or "twisting" that randomness in different ways. Also, applying it to the Bernoulli case gives the much simpler representation $X = I(U \leq q)$. – dsaxton Dec 8 '15 at 20:24

This can be accomplished using inverse transformation sampling; it does not require a restriction on the desired distribution function. To do this, define X \equiv f(\theta,\omega) \equiv \inf \{ r \in \mathbb{R} | F_\theta(r) \geqslant \omega \}$$^\dagger where F_\theta is the desired distribution function conditional on the parameter \theta. Since the distribution F_\theta is non-decreasing we have:$$\inf \{ r \in \mathbb{R} | F_\theta(r) \geqslant \omega \} \leqslant x \quad \quad \iff \quad \quad F_\theta(x) \geqslant \omega .$$Hence, taking \omega \sim \text{U}(0,1) independent of the parameter \theta you then get:$$\begin{equation} \begin{aligned} \mathbb{P}(X \leqslant x | \theta) &= \mathbb{P} \Big( \inf \{ x \in \mathbb{R} | F_\theta(x) \geqslant \omega \} \leqslant x \Big| \theta \Big) \\[6pt] &= \mathbb{P} \Big( \omega \leqslant F_\theta(x) \Big| \theta \Big) \\[6pt] &= F_\theta(x), \\[6pt] \end{aligned} \end{equation}$which is the desired distribution for$X$conditional on the parameter$\theta$. As you can see, there is no requirement on$F_\theta$for this technique to work, though it is notable that it is not necessarily the most computationally efficient technique to generate your desired random variable. This technique can be extended to multivariate problems as shown in this related question.$^\dagger$This infimum function is the "generalised inverse" of$F_\theta$. In the case where$F_\theta$is continuous you get$\inf \{ r \in \mathbb{R} | F_\theta(r) \geqslant \omega \} = F_\theta^{-1}(\omega)\$, which is the inverse in the regular functional sense.