Are National or State Polls More Reliable?

Nate Silver looks at the fact that Mitt Romney is about tied — or perhaps even has a small lead — in the average of national polls right now while President Obama leads in the key swing states.

“There are some reasons to prefer national polls to state polls. First, they probably come from slightly stronger polling firms on average and they often have larger sample sizes, although there are exceptions on either side. Second, they’re more straightforward to interpret — especially if you want to derive an estimate of how the national popular vote will break down.”

“Our research suggests, however, that when the state polls and the national polls seem to tell a different story about the state of the campaign, the state polls sometimes (not always, by any means) get it right… One is just that there are more of them… So even if the typical state poll is slightly less accurate the typical national poll, the collective sum of state polls may be more worthwhile than the collective sum of national polls. Also, the state polls come from a more diverse set of polling firms, and may provide for a greater degree of independence.”