  In a recent manuscript, Gelman & Yao (2020) claim that "the usual rules of
conditional probability fail in the quantum realm" and that "probability theory
isn't true (quantum physics)" and purport to support these statements with the
example of a quantum double-slit experiment. The present comment recalls some
relevant literature in quantum theory and shows that (i) Gelman & Yao's
statements are false; in fact, the quantum example confirms the rules of
probability theory; (ii) the particular inequality found in the quantum example
can be shown to appear also in very non-quantum examples, such as drawing from
an urn; thus there is nothing peculiar to quantum theory in this matter. A
couple of wrong or imprecise statements about quantum theory in the cited
manuscript are also corrected.
