Response rates are extremely low nowadays, and our samples, at 500 per poll, are pretty small. Some of our poll results would have been 10 points different without the ability to weight by party registration or primary vote history, and occasionally even more than 10 points different. (Weighting means giving more weight to respondents from an underrepresented group to ensure the sample reflects the demographic profile of likely voters.) In almost all of these cases, it’s Democrats who have been overrepresented, not Republicans.

Based on that, and as we wrote at the time, we decided in September to largely avoid districts without party registration or primary vote history, including some places we’d really like to poll, like Montana or Minnesota’s First and Seventh Districts. For the same reason, we also considered not re-polling Minnesota’s Eighth.

It would be foolish to rule out the possibility that this poll result would have been 10 points different if we could have weighted by party registration, given that we know it has had that kind of effect in other districts. One could find additional evidence for this case by looking at President Trump’s approval rating and the party identification of the poll, two measures that lurched far to the right even though we don’t have much reason to believe that either ought to have moved so far.

It should be noted that this problem isn’t limited to us. A lot of pollsters going without party registration will occasionally get weird results like this. To compensate, some try to weight to party identification — whether people consider themselves Democrats or Republicans.

The challenge of weighting by party identification is that it’s hard to know the “real” party identification. That’s especially true in a congressional district where we’ve done only one poll before (typically, a firm weighting by party identification will choose to weight to the average result over several previous polls of the same area).

If we had weighted to the party identification from our September poll (in which we had Democrats outnumbering Republicans by two percentage points) or the average of the two polls (R+4), the results in this survey would indeed have moved to the left.

Mr. Stauber would have led by nine points if we had weighted to the average party identification of the two polls together. He would have led by four points if we had weighted to the party identification from September.