<p>I think this one is an interesting question with an excellent answer:</p>

<p><a href="https://stats.stackexchange.com/questions/11691/how-to-tell-if-data-is-clustered-enough-for-clustering-algorithms-to-produce-me">How to tell if data is “clustered” enough for clustering algorithms to produce meaningful results?</a></p>

<p>I vote for this question because clustering is a very recurring topic, the idea behind clustering is simple but usually it is quite hard to get results in practice. </p>

<p>The question I proposed already has an accepted answer, and it deals with the gap statistic. The quality of the answer is outstanding.</p>
