What is the earliest use of the $\perp\!\!\!\!\perp$ symbol in statistics to denote statistical independence?

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?

• I occasionally see $X_1 \perp\!\!\!\!\perp \cdots \perp\!\!\!\!\perp X_n$, or similar, so the former notation is capable of illustrating the n-ary case. May 17, 2022 at 21:29
• Note that I am asking about $\perp\!\!\!\!\perp$, which has two vertical bars, rather than $\perp$ which has one vertical bar. May 17, 2022 at 21:35

An early reference that uses this notation and (at a minimum) introduced many people to this notation is the following publication.

Dawid, A. P. “Conditional Independence in Statistical Theory.” Journal of the Royal Statistical Society. Series B (Methodological) 41, no. 1 (1979): 1–31. http://www.jstor.org/stable/2984718.

Have you found earlier uses?