title: How to correctly interpret solr similarity score?
body: <p>I am aware that the similarity scores returned by <code>Solr</code> are relevant only for a specific query and that they have only relative meaning. 
  Having said that, is there a way to determine the 'goodness' of a score in a global fashion?</p>
  
  <p>For example: Suppose I run an MLT query and get 5 documents. Each document has a score but the fact is that the document with the highest score is not necessarily the most relevant. I want to be able to specify a threshold score below which I do not even consider the documents.</p>
  
  <p>How can this threshold be determined? Is it only by empirical measurement, or can I say that <em>usually</em>, a similarity score larger than 3 gives good resemblance in content, while similarity scores smaller than 1 usually means the document is completely irrelevant? Or alternatively, can I say that results that are less than 80% of the similarity of a document to itself are irrelevant?</p>