pan class="special">*</span> <span class="identifier">s</span><span class="special">.</span><span class="identifier">standard_deviation</span><span class="special">()))</span> <span class="special">*</span> <span class="number">2</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
</pre>
<p>
            To a useful precision, the 1, 2 &amp; 3 percentages are 68, 95 and 99.7,
            and these are worth memorising as useful 'rules of thumb', as,