<p>This is the output from a <a href="http://en.wikipedia.org/wiki/Social_network_analysis" rel="nofollow noreferrer">social network analysis</a>.  (I don't know much about SNA, but we have a tag for it, <a href="https://stats.stackexchange.com/questions/tagged/social-network" class="post-tag" title="show questions tagged &#39;social-network&#39;" rel="tag">social-network</a>.)  Specifically, this is an <a href="http://mathinsight.org/undirected_graph_definition" rel="nofollow noreferrer">undirected graph</a> where the sizes of the nodes are proportional to the number of questions with that tag (actually, it's probably the log or square root, etc., of the number [<em>no it isn't, note the comment from @PiotrMigdal below</em>]), and the weights of the edges is proportional to the number of questions with both tags.  By default, the colors are the result of community detection, a kind of clustering of nodes in networks.  I notice that you can change this, however.  Importantly, notice that by default these are only the top 64 tags (although this can be changed as well).  </p>

<p>Here are some specific answers:  </p>

<ol>
<li><p>I think this is pretty neat.  It gives us one way of exploring the structure of the threads on CV.  Under the (I think reasonable) assumptions that most of the threads are tagged appropriately and that the tags represent meaningful topics within statistics and machine learning, this tells us about the volume of interest (i.e., number of questions) in different topics within our purview and their interrelationships.  There seem to be five primary clusters, for example.  </p>

<p>a. A $\color{blue}{\rm modeling}$ cluster, anchored by <a href="https://stats.stackexchange.com/questions/tagged/regression" class="post-tag" title="show questions tagged &#39;regression&#39;" rel="tag">regression</a> and <a href="https://stats.stackexchange.com/questions/tagged/r" class="post-tag" title="show questions tagged &#39;r&#39;" rel="tag">r</a><br>
b. A $\color{purple}{\rm basic\ statistics}$ cluster, anchored by <a href="https://stats.stackexchange.com/questions/tagged/hypothesis-testing" class="post-tag" title="show questions tagged &#39;hypothesis-testing&#39;" rel="tag">hypothesis-testing</a> and <a href="https://stats.stackexchange.com/questions/tagged/statistical-significance" class="post-tag" title="show questions tagged &#39;statistical-significance&#39;" rel="tag">statistical-significance</a><br>
c. A $\color{green}{\rm machine\ learning}$ cluster, anchored by <a href="https://stats.stackexchange.com/questions/tagged/machine-learning" class="post-tag" title="show questions tagged &#39;machine-learning&#39;" rel="tag">machine-learning</a> and <a href="https://stats.stackexchange.com/questions/tagged/classification" class="post-tag" title="show questions tagged &#39;classification&#39;" rel="tag">classification</a><br>
d. A $\color{red}{\rm somewhat\ more\ mathematical\ take\ on\ statistics}$ cluster, anchored by <a href="https://stats.stackexchange.com/questions/tagged/probability" class="post-tag" title="show questions tagged &#39;probability&#39;" rel="tag">probability</a> and <a href="https://stats.stackexchange.com/questions/tagged/distributions" class="post-tag" title="show questions tagged &#39;distributions&#39;" rel="tag">distributions</a><br>
e. A $\color{orange}{\rm time\ series}$ cluster, composed entirely (at least within the first 64 tags) of <a href="https://stats.stackexchange.com/questions/tagged/time-series" class="post-tag" title="show questions tagged &#39;time-series&#39;" rel="tag">time-series</a> and <a href="https://stats.stackexchange.com/questions/tagged/forecasting" class="post-tag" title="show questions tagged &#39;forecasting&#39;" rel="tag">forecasting</a>  </p></li>
<li><p>The analytical methods used were based in <a href="http://faculty.ucr.edu/~hanneman/nettext/" rel="nofollow noreferrer">social network analysis</a>, no doubt built on a great deal of <a href="http://en.wikipedia.org/wiki/SQL" rel="nofollow noreferrer">SQL</a> [<em>no it isn't, note the comment from @PiotrMigdal below</em>].  Maybe someone here could do it better, but this was pretty sophisticated and I notice that there are only two CV users who are <a href="https://stats.stackexchange.com/tags/social-network/topusers">listed</a> as having answered more than one question on the <code>[S-N]</code> tag.  </p></li>
<li><p>SE could adopt this as part of, or something similar to, its <a href="http://data.stackexchange.com/" rel="nofollow noreferrer">data site</a>, if the developers wanted.  </p></li>
</ol>
