e (0, 1) interval would
have a perfect score of 0. Using a (0.1, 0.9) interval for defining ambiguity
is common as well.</p>
<p>The PAC is not, strictly speaking, an internal validity index. Originally
used to choose the optimal number of clusters, here we use it to assess
cluster stability. However, PAC is still agnostic any gold standard
clustering result so we use it like an internal validity index.</p>
    </div>
    <div class="section level2">
    <h2 id="references">References<a class="anchor" aria-l