Monmouth's national poll is suspiciously skewing toward Hillary
The Monmouth poll is regarded as a solid poll. Nate Silver's website rates it an A+ poll, but it's not apparent just why this poll receives the high grade. The poll fares worse than other pollsters in the categories he measures, yet the poll that favors Democrats more than Republicans, as noted by Silver, receives the A+ anyway.
The poll has an average margin of error of 5.5%. This is a large margin of error considering it means that a margin of eleven points must occur before a lead is outside the margin of error. That margin of error results from taking small samples. Small samples also allow for cherry picking the results a pollster might want. While that is not a certainty the smaller sample allows for it.
While the poll has some issues, it is a good poll overall, but it probably doesn't deserve an A+ rating.
Registered Voter Polls Favor Democrats Because Many Register but do not Vote
Now to the results of the latest poll. The last poll found that Hillary Clinton was ahead of Trump by thirteen points, a very large lead. Certainly that result was outside of credibility, but we can use it to judge the current poll.
The poll released this week showed a seven-point lead for Hillary, so we see that he has cut Hillary's lead in half. That is consistent with other polling. The problem, however, is that Monmouth's result for registered voters mirrored exactly the results it found for likely voters after it applied it's likely voter screen. This really should not happen much if at all.
The reason this is a red flag is that registered voter polls almost always favor Democrats. When a screen for likely voters is applied, it almost always gives the Republican a bump and the Democrat a drop. This is true because more registered Democrats stay home than do registered Republicans. Democrats are very good at registering voters, but registering them is only part of the battle.
Simply put, Democrats are less likely to actually turn out and vote. So many of the people Democrats are able to register in voter registration drives, like students for example, do not end up voting. Even though they do not vote, their answers are included in a registered voter poll, and that results in a bump for the Democrat that will not materialize on election day. Nate Silver demonstrates this exact bias using stats on his website. There's really no doubt that registered voter polls favor Democrats.
So because registered voter polls favor Democrats, one should wonder when the likely voter screen results in a +7 margin for Clinton, the same as the registered voter poll margin of +7 for Clinton. What Monmouth is telling us is that this election is different from all the others where Democrats were less likely to turn out. There's no reason to believe that is true this time around, especially considering that Republicans have registered more new people and had record turnout in its primaries.