Quinnipiac's new presidential results presaged unpleasant Senate numbers for Democrats, and indeed, that's exactly what we got. But the important thing to note is that in every case, the findings are far more favorable for Republicans than the aggregate of all available polls, which is a much better metric to rely on. (As usual, we’re relying on the HuffPost Pollster averages.) Here's what we mean:
FL: Marco Rubio (R-inc): 50, Patrick Murphy (D): 37; avg.: Rubio 43-39
FL: Marco Rubio (R-inc): 50, Alan Grayson (D): 38; avg.: Rubio 44-38
OH: Rob Portman (R-inc): 47, Ted Strickland (D): 40; avg.: Portman 42-40
PA: Pat Toomey (R-inc): 49, Katie McGinty (D): 39; avg.: Toomey 44-39
Is it possible that Quinnipiac is right and the bulk of pollsters are wrong? Certainly. But if that's the case, then you'd also have to accept, for instance, that Donald Trump leads Hillary Clinton 43-41 in Pennsylvania, even though every other poll taken this year has shown Clinton ahead. Maybe that's true, too, but that's an awfully precarious limb to climb out on.
There's another issue as well, and one we've raised before: Quinnipiac's extremely long field periods. All three surveys were conducted from June 30 through July 11, a stretch of 12 days. More typically, a horserace poll will go into the field for only three days, and with good reason. As the well-known maxim has it, a poll is a snapshot in time, and as with a camera, the longer your exposure, the blurrier your image gets.
And when you keep your lens open for nearly two weeks, breaking news developments can cloud the picture. That's especially so in this case, since FBI Director James Comey's decision not to recommend charges against Clinton over her handling of classified materials came right in the middle of Quinnipiac's field period. Did that announcement have an impact on the race? It's particularly difficult to say here, since respondents were queried for several days both before and after the story broke.
There's a reason why pollsters keep surveys in the field longer, and that's to make sure they secure a large enough sample to yield meaningful results. But the longer that takes, the more clarity you sacrifice. Quinnipiac isn't the only outfit to produce polls with long field periods, but it's uncommon, and it's always something you should be aware of when analyzing any results.