n class="special">&lt;&lt;</span> <span class="identifier">size</span><span class="special">;</span>
      <span class="comment">// calculate df for two sided test:</span>
      <span class="identifier">df</span> <span class="special">=</span> <span class="identifier">students_t</span><span class="special">::</span><span class="identifier">find_degrees_of_freedom</span><span class="special">(</span>
         <span class="identifier">fabs</span><span class="special">(</span><span class="identifier">M</span> 