Legal rules of evidence do not, of course, apply before the labor arbitrator. This is not surprising since such rules were developed in connection with jury trials, and do not apply strictly in any tribunal but a jury-court. The whole theory of the arbitration tribunal is that it is composed of experts who repeatedly inquire into a relatively homogeneous kind of cases. Exclusionary rules are hardly required as a precautionary measure. Indeed, as the late Harry Shulman said in his classic Oliver Wendell Holmes lecture at Harvard in 1955, "The more serious danger is not that the arbitrator will hear too much irrelevancy, but rather that he will not hear enough of the relevant.''
R. W. Fleming,
Some Problems of Evidence Before the Labor Arbitrator,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol60/iss2/2