There has been a lot of attention paid to the divergent polling results recently, including on this blog (here and here; see numerous posts on Pollster.com as well). As I've noted before, one reason for these divergent results may be due to the way that polling organizations are defining who is a "likely voter."
Rasmussen Reports notes the significance of these decisions when discussing their latest survey:
"It is far more challenging to project turnout in a Primary Election than a General Election. [...] The degree to which actual turnout varies from these [demographic] figures could have a significant impact on the final results."
For the most part, the survey firms appear to be coming up with relatively similar figures for the percentage of women vs. men (most are assuming about 55-60% will of the electorate will be women) and whites vs. African-Americans (most are assuming that about 80% of the electorate will be white and 15% African-American).
There does, however, appear to be some significant differences in the age breakdown of the samples being gathered by these pollsters. Unfortunately, it is sometimes difficult to compare different samples by their age breakdown because different organizations use different cut points. But let's take 3 firms where we can make comparisons.
Public Policy Polling (PPP) used an automated polling method to collect over 2,000 interviews during the weekend. They are the one firm today reporting an Obama lead (47-43%). Interestingly, the 18-45 age group made up 41% of those they polled.
On the other hand, Suffolk University conducted a poll during the same two days this weekend but they turned up a 52-42% lead for Clinton. One way to account for the major difference between these two polls is that in the Suffolk poll voters in the 18-45 age group comprised just 19% of their sample. That means that Suffolk is expecting a much older electorate tomorrow than PPP. Both firms see Obama winning the younger age group by a significant margin (49-41% in the PPP poll and 56-40% in the Suffolk survey), but Suffolk thinks that they will make up only one-fifth of the electorate while PPP believes that they will be 41% of the electorate.
Quinnipiac, who conducted interviews from Friday through Sunday, has Clinton winning Pennsylvania by a margin of 51-44%. Their poll also finds that Obama will win the 18-45 age group by a significant margin, but based on a little backwards induction, I determined that citizens between 18 and 45 years old comprised 24% of the sample.
Bottom line: pollsters do not appear to be drawing consistent samples by age and such decisions can dramatically affect the results that are being reported. PPP has Obama winning because its sample is much younger than that captured by Quinnipiac or Suffolk. The real question is, which firm is closer to having the right mix of old and young? We won't know until we see the exit polls tomorrow night, but the answer will likely determine the outcome in Pennsylvania.