De Finetti: intuition and motivation

• We have invoked the “exchangeable/iid” setup several times already
• De Finetti theorem will give us more insight in this class of models
• Motivation: often, may not want to rely on the order of the rows in the spreadsheet given to you. So we may want to shuffle it to be safe.
• In such circustance, de Finetti motivates the existence of a well-specified model with the graphical model on the right
• I.e. that there exists a prior such that the data is iid given that prior
• This motivation gives you an idea of the theorem, but is not actually 100% exactly what de Finetti says, let’s formalize the theorem!

Exchangeable random variables

• Recall: notion of equality in distribution $$X_1 {\stackrel{\scriptscriptstyle d}{=}}X_2$$
• …this means the distribution of $$X_1$$ is equal to the distribution of $$X_2$$
• …concretely: draw the marginal PMF or density of each random variable, check if they match

Poll: Are there random variables where $$X_1 {\stackrel{\scriptscriptstyle d}{=}}X_2$$ but $$X_1 \neq X_2$$?

1. This cannot happen $$X_1 {\stackrel{\scriptscriptstyle d}{=}}X_2$$ if and only if $$X_1 = X_2$$
2. You flip a fair coin and say what is on the top $$X_1$$. Your friend flip another coin $$X_2$$ with bias 1/3
3. You flip a fair coin and say what is on the top $$X_1$$. Your friend say what is on the bottom $$X_2$$
4. You flip a fair coin and say what is on the top $$X_1$$. Your friend say what is on the top, $$X_2$$
• Two random variables are exchangeable if $$(X_1, X_2) {\stackrel{\scriptscriptstyle d}{=}}(X_2, X_1)$$
• Example: indicator variable (i.e. Bernoulli random variables).

Poll: Develop a criterion for checking from the joint density if it’s exchangeable.

1. It should be invariant to 90 degree rotation around the origin (clockwise)
2. It should be invariant to 90 degree rotation around the origin (counter-clockwise)
3. It should be symmetric with respect to the line $$y = -x$$
4. It should be symmetric with respect to the line $$y = x$$

Examples of exchangeable random variables

• Using our criterion (symmetry along the line $$y = x$$), we obtain that the indicator on a square, a circle, and checkers board centered at zero all lead to exchangeable random variables.

Poll: Which one is iid (in any)

1. Only the square is iid
2. Only the circle is iid
3. Only the checker board is iid
4. All of them
5. None of them
• Note: exchangeability implies identical distributions
• Note: this notion is closely related to reversibility
• Generalization: $$n$$ variables are exchangeable if for all permutations $$\sigma : \{1, 2, \dots, n\} \to \{1, 2, \dots, n\} \in S_n$$, $$(X_1, X_2, \dots, X_n) {\stackrel{\scriptscriptstyle d}{=}}(X_{\sigma(1)}, X_{\sigma(2)}, \dots, X_{\sigma(n)})$$
• Infinite exchangeable: an infinite sequence of random variable $$(X_1, X_2, \dots)$$ is exchangeable if all finite subsets are exchangeable.

De Finetti: formal statement (vanilla version)

Theorem: if $$(X_1, X_2, \dots)$$ are exchangeable Bernoulli random variables, then there exists a $$\theta$$, a prior on $$\theta$$, and a likelihood such that the observations are iid given $$\theta$$.

• Note: This is not guaranteed to hold for finite $$n$$ (e.g., works for the checker board, not possible for the circle)
• Leads to a philosophy: “Stochastic process based design of Bayesian models”
• Construct models containing an infinite number of observations a priori
• Condition on only the subset you actually observed
• Ideally, construct the model such that you can analytically marginalize the infinite suffix of unobserved data
• Note: it is trivial to marginalization a node $$X_i$$ in a graphical model such that there is no directed path from $$X_i$$ to any observation