Target Plural or Singular words???

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When considering SEO with page titles and content should you include the plural or the singular of a wrod or a mix.

If a mix which holds more importance or does it not matter?

When looking for keyword popularity through overture it brings the same results for the plural and singular of a term, but in the search result pages for the singular and plural word are different.

Any Ideas on how to decide???

Is there any keyword tracker that recognises plurals?

At the moment Im copying what the big sites in my niche target but it would be interesting to know what ppl actually search for.

P.S I'm new to the board so Hi everyone.