# Simple functional form for correlated Bernoulli variables

I'm looking for a simple, symmetric multivariate distribution for $N$ Bernoulli variables with the following properties:

• Each individual variable takes on values 1 or 0

• Fix a subset of $M$ variables. Let $P_K$ be the probability that conditional on $K$ of these $M$ variables taking on value 1, the $N$th variable is 1. Want: $P_K$ increasing in $K$.

• A parameterized functional form with a single parameter, so that at the extremes, we arrive at the case of perfect correlation and at independence.

Hopefully this is not too simple a question to ask on mathoverflow, but I've had a tough time coming up with a simple distribution that does this. Many thanks.

• Take a multivariate Gaussian distribution on $(y_1,..,y_N)$ with a constant pairwise correlation $\rho$. Pick some threshold $T$ and set $x_i = 1$ if and only if $Y_i \geq T$ (otherwise zero). The $x_i$'s are correlated Bernoulli r.v.s. with a distribution parametrized by $\rho$ which satisfies your conditions. – Or Zuk Apr 27 '11 at 21:50

Gibbs measures provide a solution. Introduce the energy $H(x)$ of a configuration $x=(x_k)_{1\le k\le N}$ in $S=\{0,1\}^N$ as $$H(x)=\sum_kx_k+\sum_{k\ne\ell}x_kx_\ell,$$ and, for every parameter $a>0$, define a probability $P_a$ on $S$ by $$P_a(x)=a^{H(x)}/Z_a,\qquad Z_a=\sum_{x\in S}a^{H(x)}.$$ Special values are $P_1$ uniform on $S$ and $P_\infty$ the Dirac mass at the configuration all ones. For every $a>1$, the random variables $x_k$ are positively correlated due to the $x_kx_\ell$ terms in the energy $H(x)$ hence the properties you ask are satisfied.