# Single random variable, multiple probability distributions?

If we have two separate probability distributions P(x) and Q(x) over the same random variable x, we can measure how diﬀerent these two distributions are using the Kullback-Leibler (KL) divergence...

The above statement is from Deep Learning by Ian Goodfellow and Yoshua Bengio and Aaron Courville and I have the following question:

As far as I have understood, a random variable is defined considering a specific probability distribution in mind, it takes the value of a random outcome in that distribution. Perhaps I'm wrong in my understanding. My question is:

How can you have two separate probability distributions on the same random variable?

Kindly help me resolve this confusion. Thanks!

Long story short: just ignore that part ("over the same random variable $x$").

1. You are correct: a random variable is characterized by its distribution.

2. You should think of the KL divergence as measuring a "distance" between two distributions $P$ and $Q$. No need to involve random variables.

3. "over the same random variable $x$" might be referring to some of the following:

• the two distributions $P$ and $Q$ should be on the same space. (Say, if $x$ is a real number, then they should be distributions on $\mathbb{R}$. If $x$ is one of $4$ discrete values, then $P$ and $Q$ should be distributions on those $4$ values.) Note that another technical issue is that you need $P(x)=0$ for $x$ such that $Q(x)=0$, to avoid division by zero.

• maybe $x$ is some random variable on some space (see previous bullet) but with a yet-to-be specified distribution. $P$ and $Q$ are possible distributions for $x$.

The uniqueness of the distribution of a random variable $\mathbf{x}$ implicitly implies that you consider a given measure $\mathbb{P}$.

Consider a probability space $(\Omega,\mathcal{F}, \mathbb{P})$, and a measurable space $(X,\mathcal{X})$. A random variable $\mathbf{x}$ is defined on $(\Omega,\mathcal{F}, \mathbb{P})$ as a measurable function $\mathbf{x}:~\Omega \to X$. Then $\mathbb{P}_{\mathbf{x}}=\mathbb{P}\circ \mathbf{x}^{-1}$ is a measure, and classically it is defined as the distribution of $\mathbf{x}$. Consider now the probability space $(\Omega,\mathcal{F}, \mathbb{Q})$, and the same function $\mathbf{x}:~\Omega \to X$. Then $\mathbb{Q}_{\mathbf{x}}=\mathbb{Q}\circ \mathbf{x}^{-1}$ is a also a measure. Therefore, if you define the random variable as a function, without a specific measure but only considering the measurable space $(\Omega,\mathcal{F})$, two different measures will give two different distributions. The KL divergence compares two measures, for a single measurable function $\mathbf{x}$.

However classically random variables are defined for a given probability measure $\mathbb{P}$, therefore have a given distribution $\mathbb{P}_{\mathbf{x}}$.

Ideally there is only one "correct" probability measure for any real-world process.   However, there may be many competing theoretical models for that process.   So we need tools to compare the probability distributions of those models ; to find the best fit to experimental results and such.

That is roughly what you are dealing with.