wolfram.com/CorrelationCoefficient.html">
 Pearson's product moment correlation coefficient</a>,
 usually denoted r.
 <p>
 <strong>Preconditions</strong>: <ul>
 <li>At least two observations (with at least two different x values)
 must have been added before invoking this method. If this method is
 invoked before a model can be estimated, <code>Double,NaN</code> is
 returned.
 </li></ul></p></div>
<dl><dt><span class="strong">Returns:</span></dt><dd>Pearson's r</dd></dl>
</li>
</ul>
<a name="getRSquare()">
