Yesterday I visited a journalism class. The question arose of how to interpret the L.A. Times/Dornsife/USC poll, which has been unusually favorable to Donald Trump. I said that polls should be treated the way reporters treat other sources of information: get confirmation from a second source. In the case of polls, find two other sources and take the median. The reason is that a polling result is not a pure number descended from heaven; it reflect the professional judgment of one pollster…and such judgments can vary.

Because the Dornsife/USC poll is an outlier compared to other surveys, its other impressive qualities are often overlooked. Let me get into those a little bit.

The Dornsife survey tracks the same respondents over people repeatedly in time, which reduces swings due to respondent bias. This is a great help, since nonresponder bias is often claimed to create false swings in the polls.

Dornsife’s unusual result appears to arise from weighting that is more detailed than what other pollsters do. This “microweighting” has been claimed to give unrepresentative results, for example because of one 19-year-old respondent in Chicago who represents his entire demographic. For the record, the L.A. Times people do not think this is such a big problem.

Luckily, Dornsife has done something remarkable: they have made their detailed response data publicly available. Using this data, Ernie Tedeschi has used Census/American Community Survey demographics to reweight the responses. Perhaps unsurprisingly, the resulting output more closely resembles polling averages. See the graph above.

None of this is to criticize the Dornsife methods. In fact, those methods provide valuable evidence that the small swings in the race this year are in fact real. I think it is one of the more interesting surveys this year. I just don’t think it means that Trump has ever been ahead in the general election, an idea that is contradicted by other polling evidence.

As readers know, I do not think it is appropriate to second-guess pollster methods. It is too easy to only do so when results are disagreeable. For this reason, here at PEC I report poll medians based on data reported by pollsters. The assumption is to respect their judgment as a whole. In this case, I think that Tedeschi’s analysis is interesting because it gives a salient case study in the judgments that all pollsters must make.