New Hampshire polls show mostly harmless noise

On Tuesday, a new Granite State Poll of the two House races in New Hampshire was released. The results showed Democrat Carol Shea-Porter with a three-point lead in NH-01, and Republican Marlinda Garcia with a four-point lead in NH-02.

Here's how those results look in the context of previous polls from the same pollster this year:

Wow! Those are two very dramatic races—up, down, and back again.

Well, maybe not so much. The problem? The number of voters in these surveys has ranged from a high of 307, to just 184.

This means that random error alone, excluding all the other delightful and varied sources of error in polling, can account for the vast majority of the movement seen in the polling numbers.

To demonstrate this, I simulated a poll of a population with an actual margin of zero (a tie) 50 times in a row (with no undecideds). I used only 250 respondents. This is the result:

The simulated poll bounces around like crazy, and in about the same range as the polls we see from UNH. In other words, a poll with only 250 respondents is utterly useless if you want to follow changes in voter preference over time for all but the most dramatic races.

Please donate $3 to help Carol Shea-Porter win regardless of these bouncy polls!

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But on the bright side, the way to get rid of this crazy behavior is simple (but costly—increase your sample size. Follow me below the fold to see how this changes the behavior of the simulated poll.