 getSlope() + getSlopeConfidenceInterval())</code></p>
 <p>
 If there are fewer that <strong>three</strong> observations in the
 model, or if there is no variation in x, this returns
 <code>Double.NaN</code>.</p>
 <p>
 <strong>Usage Note</strong>:<br>
 The validity of this statistic depends on the assumption that the
 observations included in the model are drawn from a
 <a href="http://mathworld.wolfram.com/BivariateNormalDistribution.html">
 Bivariate Normal Distribution</a>.</p></div>
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