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diff --git a/src/main/java/org/apache/commons/math3/stat/descriptive/moment/SecondMoment.java b/src/main/java/org/apache/commons/math3/stat/descriptive/moment/SecondMoment.java
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+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.commons.math3.stat.descriptive.moment;
+
+import java.io.Serializable;
+
+import org.apache.commons.math3.exception.NullArgumentException;
+import org.apache.commons.math3.util.MathUtils;
+
+/**
+ * Computes a statistic related to the Second Central Moment.  Specifically,
+ * what is computed is the sum of squared deviations from the sample mean.
+ * <p>
+ * The following recursive updating formula is used:</p>
+ * <p>
+ * Let <ul>
+ * <li> dev = (current obs - previous mean) </li>
+ * <li> n = number of observations (including current obs) </li>
+ * </ul>
+ * Then</p>
+ * <p>
+ * new value = old value + dev^2 * (n -1) / n.</p>
+ * <p>
+ * Returns <code>Double.NaN</code> if no data values have been added and
+ * returns <code>0</code> if there is just one value in the data set.
+ * Note that Double.NaN may also be returned if the input includes NaN
+ * and / or infinite values.</p>
+ * <p>
+ * <strong>Note that this implementation is not synchronized.</strong> If
+ * multiple threads access an instance of this class concurrently, and at least
+ * one of the threads invokes the <code>increment()</code> or
+ * <code>clear()</code> method, it must be synchronized externally.</p>
+ *
+ */
+public class SecondMoment extends FirstMoment implements Serializable {
+
+    /** Serializable version identifier */
+    private static final long serialVersionUID = 3942403127395076445L;
+
+    /** second moment of values that have been added */
+    protected double m2;
+
+    /**
+     * Create a SecondMoment instance
+     */
+    public SecondMoment() {
+        super();
+        m2 = Double.NaN;
+    }
+
+    /**
+     * Copy constructor, creates a new {@code SecondMoment} identical
+     * to the {@code original}
+     *
+     * @param original the {@code SecondMoment} instance to copy
+     * @throws NullArgumentException if original is null
+     */
+    public SecondMoment(SecondMoment original)
+    throws NullArgumentException {
+        super(original);
+        this.m2 = original.m2;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void increment(final double d) {
+        if (n < 1) {
+            m1 = m2 = 0.0;
+        }
+        super.increment(d);
+        m2 += ((double) n - 1) * dev * nDev;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public void clear() {
+        super.clear();
+        m2 = Double.NaN;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public double getResult() {
+        return m2;
+    }
+
+    /**
+     * {@inheritDoc}
+     */
+    @Override
+    public SecondMoment copy() {
+        SecondMoment result = new SecondMoment();
+        // no try-catch or advertised NAE because args are guaranteed non-null
+        copy(this, result);
+        return result;
+    }
+
+    /**
+     * Copies source to dest.
+     * <p>Neither source nor dest can be null.</p>
+     *
+     * @param source SecondMoment to copy
+     * @param dest SecondMoment to copy to
+     * @throws NullArgumentException if either source or dest is null
+     */
+    public static void copy(SecondMoment source, SecondMoment dest)
+        throws NullArgumentException {
+        MathUtils.checkNotNull(source);
+        MathUtils.checkNotNull(dest);
+        FirstMoment.copy(source, dest);
+        dest.m2 = source.m2;
+    }
+
+}