Frequency is how often a keyword appears on the page or in an area on the page. In general, the more times a keyword appears on the page, the more relevant it will be to that search.
You don't want to go overboard with frequency since many engines will penalize you for keyword "spamming" if they feel you were excessive. In general though, use your keyword in the document in as many different areas as you can, and as many times as is recommended for that engine.
Frequency counts will often vary depending on whether you selected exact matching on the first tab of the Page Critic screen. Frequency will also vary depending on whether you enable partial matching. In addition, if you perform a case-sensitive search by including one or more upper case letters in your keyword, then your frequency count may vary.
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