How can that be? I’ll get to the math in a moment, but let me give you the implications first. According to my analysis, if Jane Doe is polling at 30 percent in Iowa and 10 percent nationally, she would be expected to win 34 percent when the final Iowa results are tallied.1 Meanwhile, if Jane Doe is polling at 30 percent in Iowa and 30 percent nationally, she would be expected to get only 29 percent of the Iowa vote.

That dynamic is evident in real-world results too. Take the 2008 primary campaigns: Mike Huckabee averaged 22 percent in Iowa in the second to last month before the caucus but 8 percent nationally. He went on to earn 34 percent of the Iowa vote. Rudy Giuliani was polling at 13 percent in Iowa and 28 percent nationally, and finished with just 4 percent of the Iowa vote. On the Democratic side that year, Barack Obama was polling at 25.5 percent in Iowa and 22.5 percent nationally, and ended up with 35 percent of the Iowa vote.

Of course, this result could just be statistical noise. That is, the result could just be happening by chance. We’ve only had 12 primary campaigns without an incumbent president running since 1980 (the sample I used) — that’s not a ton of data.

Still, there are some common-sense ways to explain why it may be better to do worse in national polls.