= Undisclosed qubit measurement outcome with an unspecified preparation prior
{title2=$\rho=\operatorname{diag}(q,1-q)$}
After a <Computational basis>[computational-basis] <projective measurement>, an observer who lacks the result assigns a diagonal <density matrix>. If the observer's preparation prior has mean $q=\mathbb E|\alpha|^2$, the assignment is the displayed <matrix>. No outcome record and no stated prior do not logically force $q=1/2$; that value follows from a symmetric or maximum-entropy ignorance convention. An observer who knows the preparation instead uses $q=|\alpha|^2$.
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