Explanation by Tom Jensen from PPP:
"Three days ago we said Barack Obama was up four in Colorado. Today Rasmussen says John McCain is up a point. I don't know who's right but I can tell you one reason for the difference in our polls.
Rasmussen's numbers are based on 500 interviews conducted Wednesday night.
We did 500 interviews last Tuesday night in Colorado, and based on those McCain was up by a point as well. But we also called everyone in the sample who we didn't reach Tuesday night again Wednesday morning, Wednesday afternoon, Wednesday night, and Thursday afternoon. We got almost another 500 respondents and they went for Obama by a good amount, pushing him to a four point lead.
As far as I can tell Rasmussen just makes calls until they get 500 respondents. When we buy a sample we attempt to reach every number in it at least four times over a period of several days and go with whatever number of respondents we get based on that. And we almost always find that the further into conducting the poll we go, the better Obama does. I don't know if it's that his supporters are harder to get on the line or what, but we're finding that trend in nearly every state we poll.
I'm not saying that one way or another is better- Rasmussen has an outstanding track record- just pointing out that differences in the way we conduct our polls may help explain some of the differences in the numbers we produce. It's not as simple as just saying Rasmussen is Republican and PPP is Democratic."
http://publicpolicypolling.blogspot.com/2008/08/ppp-rasmussen-and-colorado.html
