CBS offers us another instant reaction poll that suggests a modest bump in support for health reform. The methodology they employed is worth reviewing because it is suited for assessing shifts in opinion around a major event.
Rather than conduct a one-night poll with a fresh sample, as Gallup did, to try to get a handle on immediate reactions to the passage of health care reform, CBS opted to attempt to re-contact the 1,049 adults over the last two nights that they originally interviewed last week. They were able to interview 649 adults for a second time.
The big advantage of this sort of interview, something typically called a "panel-back," is that it allows the pollster to ask the same respondents identical questions and look at change on an individual level. Also, the pollsters know whether the missing 400 respondents introduced any sort of skew in the demographics or attitudes on politics or health reform (based on responses to the first survey), and they can attempt to correct any such bias with weighting.
The potential disadvantages of this sort of survey are the attrition of original respondents and the possibility that the experience of being interviewed the first time affected the original sample somehow, altering their views of the issue or perhaps making them more likely to seek out news over the last few days as a result of their first interview (see this column for more on panel-back surveys).
The bottom line is that CBS found a nine-point net shift Obama's approval rating on health care (from 41%-51% approve-disapprove before the vote to 47%-48% approve-disapprove after) and a more modest shift in approval of the reform bill itself. A plurality still disapprove of the bill (42% approve, 46% disapprove), but the margin is narrower than before passage (37% approve, 48% disapprove).
The CBS analysis adds:
It may take more time before Americans decide whether or not these reforms are advantageous to them personally, and the percentage that now thinks the bill won't affect them has grown. When re-interviewed, 43% say the reforms will not have much of an affect on themselves or their families, up eight points from 35% before the vote. Just 16% say the legislation will help them personally - down four points from before the vote.