Out & About Blogger To McWherter: Your Poll Is Too Small

By Kleinheider Posted on November 19, 2009 at 9:34 am

From Sam Jones:

The McWherter campaign’s poll had a sample size of 602 Democratic primary voters. I wanted to find out if that number is adequate to generalize about Democrats in Tennessee. So, enter a general population size of 298,466 (the number of people who voted in the gubernatorial primary in 1998), enter a margin of error (or confidence interval) of 3%, and enter a confidence level of 95%. The McWherter campaign’s poll, if calculated in 1998, should have had a sample size of 1,063.

The McWherter campaign’s poll had a sample size of 602, though.

That would be for 1998, and one can assume that the number of Democratic primary voters in Tennessee has grown since 1998.

The McWherter campaign’s poll does not have a 3% margin of error (more likely a 4%), and we can not even be 95% sure of its results at this margin of error.

To be 99% confident in the McWherter poll with a +/- 3% margin of error, the McWherter poll would have had to have called at least 1,838 Democratic primary voters, almost three times the sample size of the poll.

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