title: Standard deviation with Apache Commons Math
body: <p>I am computing the SD of a vector using Apache Commons Math.  The problem:  I get different values than by hand</p>
  
  <pre><code>DescriptiveStatistics stats = new DescriptiveStatistics();
  stats.addValue(value1);
  ...
  stats.addValue(value8);
  stats.getStandardDeviation();
  </code></pre>
  
  <p>E.g., take the values [1699.0, 1819.0, 1699.0, 1719.0, 1689.0, 1709.0, 1819.0, 1689.0].  SD should be 52.067 but Commons Math = 55.662.</p>
  
  <p>What am I doing wrong?</p>