See: this article. Unfortunately, they do not describe the source of the bias.
Ok --- here are the actual poll results.
Here's the details they give:
This poll was conducted by telephone on May 11-13, 2012 among 615 adults nationwide, including 562 registered voters, who were first interviewed for a CBS News/New York Times Poll conducted April 13-17, 2012. Phone numbers were dialed from samples of both standard land-line and cell phones. The error due to sampling for results based on the entire sample could be plus or minus four percentage points. The margin of error for the sample of registered voters is four percentage points. The error for subgroups is higher. This poll release conforms to the Standards of Disclosure of the National Council on Public Polls.
There are a couple of things missing here. First of all, it's not clear how many voters "dropped off" from the first panel. Secondly, they don't warn readers that answers from respondents in a second survey might be quite different than from someone taking a survey for the first time. (What if the previous survey contained a question about gay marriage?).
This is an important issue. To think that the President changed his position for political reasons, someone needed to have followed politics closely enough for the previous weeks to have that context. That sort of increased interest in politics is precisely the sort of thing we'd expect out of someone who:
- Took a poll already
- Agreed to take another political survey
So the suggestion of bias is not completely out of place.
--- Stag Staff