What does AC Nielsen 52 Weeks mean?

I found it on an individual serving packet of Lipton tea. It says “*AC Nielsen 52 weeks ending March 17, 2012”

AC Nielsen in an independent research company that ranks things.

The little note is probably because somewhere else it says something like “The Best Selling Tea.” and they need to provide the source of that statement.
What Is Ac Nielsen
It means that thru a independent study..AC NIELSON(study survey company) found them as americas favorite tea march 17 2012 study
is that mean experation date
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