[nj26]

"This study is worthless. It only interviewed about 500 people and many of them are "non-voters" and as far as we can tell they are probably not even citizens in the first place."

The only meaningful statistic is the opinion of registered voters.

[old republic]

The only meaningful statistic is the opinion of registered voters.

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I would amend your statement to read:

The only meaningful statistic is the opinion of registered voters who vote.