The results could make your head spin if you believe that Biden is well ahead.
Here’s the truth that comports with the polling: Biden is ahead, but the race has been and will likely remain within the margin of error until Election Day.
Of course, each of these polls come with a sampling margin of error. Both polls had reported margins of error of a little less than +/- 5 points. That margin of error, though, applies to each of the candidates, not the margin between them. For the ABC News/Washington Post poll, this means that 95% of the time (i.e. the 95% confidence interval), Trump’s true percentage is somewhere between about 46% and 56%.
In the case of the Monmouth poll, Biden’s 5-point lead could realistically be a deficit of around 4 points or a lead in the double-digits. Likewise, the ABC News/Washington Post poll could realistically be a lead for Trump in the double digits or a Biden advantage of around 4 points.
Usually, the actual state of the race is in-between what the best polls for one candidate are and the best polls for the other candidate.
In Florida, the average has Biden leading by around 2 points.
Such a spread is, as in the case of Florida, a signal that pollsters are doing their job.
The average of all the Arizona polls has Biden up by around 4 to 5 points.
We look to the average because it’s most often closest to the truth. Chances are Biden is leading in both Arizona and Florida.
Since 1972, the 95% confidence interval for polling averages in close races has been closer to +/- 9 points. I call this confidence interval the “true margin of error” because it takes into account more than just sampling error. Other errors include the potential for the types of voters turning out being different than what pollsters anticipate (e.g. more young people than expected casting a ballot). Another potential cause for a pollster misfire is that one candidate’s supporters are less willing to answer polls.
The bottom line is that there are a slew of errors that the traditional reported margin of error doesn’t take into account.
The polls though could also be underestimating Trump. Biden’s advantage in the states that will likely determine whether he wins or loses the election is only about 5 points.
Even if Biden maintains his current lead, past errors indicate Trump will still have a non-nominal chance to pull off the victory.