Update, post-primary: Sanders outperformed his final New Hampshire polling margin by about four percentage points. That is interesting, but not out of bounds. It may reflect the difficulty of polling an open primary. It remains obviously true that Iowa did not have effects of a size to affect the likelihood of a Sanders win in New Hampshire.
It is probably hopeless to counteract reporters who weave the tale that Sanders is surging. Based on actual data, it might be exactly the opposite.
- In aggregated data, Hillary Clinton has gotten approximately a 6-point bounce in New Hampshire. The median margin was Sanders +21.5% in 4 surveys conducted January 26-30. This narrowed to Sanders +15.5% in 6 surveys conducted February 2-5.
- A daily tracking poll from U.Mass. Lowell shows even more narrowing. On February 1 it showed Sanders +31%, which by February 6th narrowed to Sanders +14%, a 17 percentage point change in Clinton’s favor.
- In national surveys, Clinton went from a median of Clinton +12% (4 polls, January 22-February 1) to Clinton +16% (3 polls, February 2-4). This is noisy data, but the median change is a national 4-point bounce for Clinton. It is possible there was little change in either direction (see confidence intervals below).
It is likely that Hillary Clinton has remained level or risen in her national standings. This may be counterintuitive, considering the tone of the coverage by the press corps. Since Sanders is still in the lead in New Hampshire, my guess is that few reporters will get static for portraying an imagined surge for him.
Josh Marshall is all up in arms about the Quinnipiac poll showing a near-tie. He doesn’t highlight the fact that three national polls done at nearly the same time show a wide range of results, including a result showing Clinton leading Sanders, 53% to 32%.
Then again, the Quinnipiac data point is interesting. it shows Sanders at 42%, which along with an online Reuters surveys showing him at 43%, are his highest numbers to date. These are likely-voter surveys and they may be measuring increased enthusiasm for Sanders. If true, we would have the interesting phenomenon that his movement in New Hampshire and nationally are in opposite directions. That could be because people in New Hampshire know him already, and noticed the Clinton win; while nationally, some are newly cueing in to him.
As Matt McIrvin puts it in comments: “I think [movement in both directions is] entirely plausible. In both cases, it’d be a shrinking lead as the respective demographics start considering the other candidate.”
[Confidence intervals: In New Hampshire, I estimate that Clinton's bounce was between -0.3 and +12.3% (one-sigma confidence interval). Nationally, the post-Iowa shift was between 2.4% toward Sanders and 10.4% toward Clinton. Overall, it seems that she gained from her narrow win in Iowa.]
P.S. Please, no comments about individual polls being biased in their likely-voter screens and so on. It is nearly impossible to do a good job of evaluating such claims without falling prey to one’s own motivated reasoning. This is why one takes the median of all legitimate polls – to reduce the problems contributed by extreme individual measurements.