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diff --git a/boost/histogram/accumulators/mean.hpp b/boost/histogram/accumulators/mean.hpp
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+// Copyright 2015-2018 Hans Dembinski
+//
+// Distributed under the Boost Software License, version 1.0.
+// (See accompanying file LICENSE_1_0.txt
+// or copy at http://www.boost.org/LICENSE_1_0.txt)
+
+#ifndef BOOST_HISTOGRAM_ACCUMULATORS_MEAN_HPP
+#define BOOST_HISTOGRAM_ACCUMULATORS_MEAN_HPP
+
+#include <boost/histogram/fwd.hpp>
+#include <cstddef>
+#include <type_traits>
+
+namespace boost {
+namespace histogram {
+namespace accumulators {
+
+/** Calculates mean and variance of sample.
+
+  Uses Welfords's incremental algorithm to improve the numerical
+  stability of mean and variance computation.
+*/
+template <class RealType>
+class mean {
+public:
+  mean() = default;
+  mean(const std::size_t n, const RealType& mean, const RealType& variance)
+      : sum_(n), mean_(mean), sum_of_deltas_squared_(variance * (sum_ - 1)) {}
+
+  void operator()(const RealType& x) {
+    sum_ += 1;
+    const auto delta = x - mean_;
+    mean_ += delta / sum_;
+    sum_of_deltas_squared_ += delta * (x - mean_);
+  }
+
+  template <class T>
+  mean& operator+=(const mean<T>& rhs) {
+    const auto tmp = mean_ * sum_ + static_cast<RealType>(rhs.mean_ * rhs.sum_);
+    sum_ += rhs.sum_;
+    mean_ = tmp / sum_;
+    sum_of_deltas_squared_ += static_cast<RealType>(rhs.sum_of_deltas_squared_);
+    return *this;
+  }
+
+  mean& operator*=(const RealType& s) {
+    mean_ *= s;
+    sum_of_deltas_squared_ *= s * s;
+    return *this;
+  }
+
+  template <class T>
+  bool operator==(const mean<T>& rhs) const noexcept {
+    return sum_ == rhs.sum_ && mean_ == rhs.mean_ &&
+           sum_of_deltas_squared_ == rhs.sum_of_deltas_squared_;
+  }
+
+  template <class T>
+  bool operator!=(const mean<T>& rhs) const noexcept {
+    return !operator==(rhs);
+  }
+
+  std::size_t count() const noexcept { return sum_; }
+  const RealType& value() const noexcept { return mean_; }
+  RealType variance() const { return sum_of_deltas_squared_ / (sum_ - 1); }
+
+  template <class Archive>
+  void serialize(Archive&, unsigned /* version */);
+
+private:
+  std::size_t sum_ = 0;
+  RealType mean_ = 0, sum_of_deltas_squared_ = 0;
+};
+
+} // namespace accumulators
+} // namespace histogram
+} // namespace boost
+
+#ifndef BOOST_HISTOGRAM_DOXYGEN_INVOKED
+namespace std {
+template <class T, class U>
+/// Specialization for boost::histogram::accumulators::mean.
+struct common_type<boost::histogram::accumulators::mean<T>,
+                   boost::histogram::accumulators::mean<U>> {
+  using type = boost::histogram::accumulators::mean<common_type_t<T, U>>;
+};
+} // namespace std
+#endif
+
+#endif