 NaN

NaN, meaning “not a number”, is a special datum in floating-point arithmetic used for an invalid real operation or a propagated undefined result. Examples include $0/0$ and the real square root of a negative number. It is distinct from the positive or negative infinity produced by some other exceptional operations.

Ordinary ordered comparisons involving NaN are false, and even equality of NaN with itself is false; use a dedicated NaN test rather than equality to detect it. A missing numerical estimate or a displayed NaN is consequently not a numerical zero and cannot serve as a valid p-value.^[ref]