The symbol $\perp\!\!\!\!\perp$ in statistics is a way to denote statistical independence of a collection of random variables. I have seen two forms of it. The first is highly suitable in writing about independence between two variables:
$$X\perp\!\!\!\!\perp Y$$
The second works for two variables, but is especially suited for multiple variables:
$$\perp\!\!\!\!\perp \left[ X_1, \cdots, X_n \right]$$
What these symbols mean, for a collection of (let's say real-valued) random variables $\{ X_j \}_{j=1}^n$, is
$$\perp\!\!\!\!\perp \left[ X_1, \cdots, X_n \right] \iff F_{X_1, \cdots, X_n} (x_1, \cdots, x_n) = \prod_{j=1}^n F_{x_j}(x_j)$$
where $F_{X_1, \cdots, X_n}$ is the joint cumulative distribution function and the $F_{x_j}$ is the marginal cumulative distribution function for the $j$th random variable.
What is the earliest-known publication that used the $\perp\!\!\!\!\perp$ symbol (either as a binary or n-ary relation) to denote statistical independence between/among random variables?
