="special">);</span>
</pre>
<p>
      If the data has variance σ<sup>2</sup>, then the variance of the computed derivative
      is roughly σ<sup>2</sup><span class="emphasis"><em>p</em></span><sup>3</sup> <span class="emphasis"><em>n</em></span><sup>-3</sup> Δ
      <span class="emphasis"><em>t</em></span><sup>-2</sup>, i.e., it increases cubically with the approximation
      order <span class="emphasis"><em>p</em></span>, linearly with the data variance, and decreases
      at the cube of the filter l