So I was reading through some forum postings on SEO and came across a question on highrankings.com about plurals in keywords.  Everyone agreed in the forum posts that including plurals in your keywords will give more accurate search results. 

They recommend testing it yourself.  if you search for “search engine” and search for “search engines” you will get completely different results. 

I poked around some more, and found this great article on searchengineguide.com about plural vs. singular keywords.  Sumantra Roy outlined how each search engine handles the difference between singular and plural keywords.  And all twelve search engines reviewed have different results for singular and plural keyword searches. 

So there you have it.  When building a list of keywords, include singular and plural versions of keywords.