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boost::accumulators::impl::variance_impl — Iterative calculation of variance.
// In header: <boost/accumulators/statistics_fwd.hpp> template<typename Sample, typename MeanFeature, typename Tag> struct variance_impl : public accumulator_base { // construct/copy/destruct template<typename Args> variance_impl(Args const &); // public member functions template<typename Args> void operator()(Args const &); result_type result(dont_care) const; template<typename Archive> void serialize(Archive &, const unsigned int); };
Iterative calculation of sample variance  according to the formula
 according to the formula 
 where 
 is the estimate of the sample mean and  is the number of samples.
 is the number of samples.
Note that the sample variance is not defined for  .
.
A simplification can be obtained by the approximate recursion
 because the difference 
 converges to zero as  . However, for small
. However, for small  the difference can be non-negligible.
 the difference can be non-negligible.