If there is one thing that I have found to be an absolute truism as a pollster in this year's election, it's that Barack Obama's supporters are harder to get in touch with than John McCain's and Hillary Clinton's.
During the early stages of the primary we released a lot of one night, one shot polls and consistently underestimated Obama by a whole heck of a lot (even if we still did better than a lot of our colleagues in places like South Carolina.)
So we started doing longer field periods and more callbacks starting with Wisconsin, and our polls were pretty much on the mark for the rest of the primary season (with of course one exception.)
This trend has continued in the general election. We conducted half the interviews for our Florida poll this week last Thursday night. The first night McCain led by 2. The data based on callbacks over the next three days gave Obama a 4 point lead.
It was a similar story in Colorado two weeks ago. The first night Obama led by 5. The rest of the field period he led by 15.
I don't think there was movement in Obama's direction in subsequent days either of those times. It's simply a matter that you have to try more times to get his supporters on the phone.
Which leads me to today's Quinnipiac Ohio poll showing Obama up by 14. That's a big difference from Rasmussen and Mason Dixon's polls earlier in the week that showed McCain winning, and I think part of the discrepancy can be traced back to the length of field time. Rasmussen's was a one night poll. Mason Dixon's was a two day poll. They couldn't have done that many, or any, callbacks to get folks on the line who weren't at home on the first shot. Quinnipiac was in the field for six days, and I'm guessing they managed to get a lot more of those elusive Obama supporters polled when they tried for the third or fourth time.
Another thing to consider when looking at polls is the number of respondents- not just whether it's a large or small number, but whether it's a nice even number or not. If it's a nice even number a lot of the time that means a pollster is just getting a set number of interviews and then stopping. That may have the potential to undercount support for Obama, again because his folks are harder to find. If it's a random number, that's an indication that folks have their sample and they've decided they're going to call everyone in it 'x' number of times and try to get as many respondents as they can get.
We're not going to be putting out Quinnipiac sort of numbers on Ohio this weekend: more like Suffolk numbers. But from my experience anyway the issue of field periods and round numbers may help to explain at least some of the 16 point difference you see between Rasmussen and Quinnipiac.