this is guaranteed to return zero if the the dataset contains only exactly one finite value.it is not guaranteed to return zero when the dataset consists of the same value multiple times,due to numerical errors. however, it is guaranteed never to return a negative result.
non-finite values
if the dataset contains any non-finite values ({@link double#positive_infinity},{@link double#negative_infinity}, or {@link double#nan}) then the result is {@link double#nan}.
"public final double populationStandardDeviation()"