at <strong>three</strong> observations in the
 model, or if there is no variation in x, this returns
 <code>Double.NaN</code>.
 </li>
 <li><code>(0 < alpha < 1)</code>; otherwise an
 <code>OutOfRangeException</code> is thrown.
 </li></ul></p></div>
<dl><dt><span class="strong">Parameters:</span></dt><dd><code>alpha</code> - the desired significance level</dd>
<dt><span class="strong">Returns:</span></dt><dd>half-width of 95% confidence interval for the slope estimate</dd>
<dt><span class="strong">Throws:</s