Add conclusion to Relevancy chapter

170_Relevance.asciidoc

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 include::170_Relevance/75_Changing_similarities.asciidoc[]
 
+include::170_Relevance/80_Conclusion.asciidoc[]

170_Relevance/80_Conclusion.asciidoc

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+
+[[relevance-conclusion]]
+=== Relevance tuning is the last 10%
+
+In this chapter, we looked at a how Lucene generates scores based on TF/IDF. 
+Understanding the score generation process is critical so you can
+tune, modulate, attenuate and manipulate the score for your particular
+business domain.
+
+In practice, simple combinations of queries will get you good search results. 
+But to get _great_ search results, you'll often have to start tinkering with
+the previously mentioned tuning methods.
+
+Often, applying a boost on a strategic field, or re-arranging a query to 
+emphasize a particular clause, will be sufficient to make your results great.
+Sometimes you'll need more invasive changes.  This is usually the case if your
+scoring requirements diverge heavily from Lucene's word-based TF/IDF model (e.g. you
+want to score based on time or distance).
+
+With that said, relevancy tuning is a rabbit-hole that you can easily fall into 
+and never emerge.  The concept of "most relevant" is a nebulous target to hit and
+different people often have different ideas about document ranking.  It is easy 
+to get into a cycle of constant fiddling without any apparent progress.
+
+We would encourage you to avoid this (very tempting) behavior and instead properly
+instrument your search results.  Monitor how often your users click the top
+result, the top-ten, the first page, how often they execute a secondary query
+without selecting a result first, how often they click a result and immediately
+go back to the search results, etc etc.
+
+These are all indicators of how relevant your search results are to the user.
+If your query is returning highly relevant results, the user will select one of
+the top five results, find what they want and leave.  Non-relevant results cause
+the user to click around and try new search queries.
+
+Once you have instrumentation in place, tuning your query is simple.  Make a change,
+monitor it's effect on your users, repeat as necessary.  The tools outlined in this
+chapter are just that: tools.  You have to use them appropriately to propel
+your search results into _great_ category, and the only way to do that is with
+strong measurement of user behavior.