<p>My initial hunch is that we may be OK with the tags we already have.  I would think we would do best to tag the threads you describe with something like one of the following:  </p>

<ol>
<li><a href="https://stats.stackexchange.com/questions/tagged/binary-data" class="post-tag" title="show questions tagged &#39;binary-data&#39;" rel="tag">binary-data</a> and <a href="https://stats.stackexchange.com/questions/tagged/correlation" class="post-tag" title="show questions tagged &#39;correlation&#39;" rel="tag">correlation</a>, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;rather than <code>[biserial]</code> and <code>[correlation]</code></li>
<li><a href="https://stats.stackexchange.com/questions/tagged/ordinal" class="post-tag" title="show questions tagged &#39;ordinal&#39;" rel="tag">ordinal</a> and <a href="https://stats.stackexchange.com/questions/tagged/correlation" class="post-tag" title="show questions tagged &#39;correlation&#39;" rel="tag">correlation</a>, &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;rather than <code>[polyserial]</code> and <code>[correlation]</code></li>
<li><a href="https://stats.stackexchange.com/questions/tagged/categorical-data" class="post-tag" title="show questions tagged &#39;categorical-data&#39;" rel="tag">categorical-data</a> and <a href="https://stats.stackexchange.com/questions/tagged/correlation" class="post-tag" title="show questions tagged &#39;correlation&#39;" rel="tag">correlation</a>, &nbsp;&nbsp;etc.</li>
</ol>

<p>I wouldn't want to have special tags for these (e.g., <code>[point-biserial-correlation]</code>), because I think it would be best to still have such treads categorized under the existing <code>[correlation]</code> tag, and using both of those would strike me as weird and redundant.  That does leave the option of just, say, <code>[biserial]</code> and <code>[correlation]</code>, but if we are going to add two tags to a thread, we already have tags that strike me as natural.  Creating new tags that will sometimes be used and sometimes the existing tags will be used seems to me likely to lead to the information on the site being less well organized rather than better.  </p>
