Early voting and vote by mail are about to end and the reports so far seem to indicate that these early and mail voters will exceed the numbers from 2010. This has lead a number of analysts to try to crunch the numbers and figure out which candidate has the advantage from early voting, something that can't be done unless you know the voting history of each voter. The campaigns and the state parties have this data, pundits typically do not, so the only thing you can do is put together a list of good questions to ask these campaigns/parties and hope it will give you useful information. Let me explain.
I vote in every election, I am a habitual voter. The sun will rise in the east,
the Cubs will not win the World Series and I will vote. No campaign should spend any money or effort trying to get me out to vote, that would be like spending money to remind me to breathe. Last week I voted early, as I have in every election since early voting became legal. I didn't vote early because of any candidate's GOTV operation, I voted early because that's what I do.
If you want to measure which campaign's GOTV operation is performing the best you need to be able to separate out which voters are habitual voters vs. which voters are infrequent voters and early/mail voting GOTV targets. The campaigns have each defined a universe of infrequent voters that are very likely to support their candidate if they make it to the polls and they're working like crazy to GOTV these voters with either early vote or vote by mail. In order to correctly measure which side is performing best for early/mail voting you need to isolate and measure these efforts.
The campaigns divide voters into three categories 1) will vote for my candidate, 2) will vote for the opponent, 3) don't know. The size of the first two groups is much bigger than you would probably expect. In the old days they would determine their support group (run universe) from the list of people who've pledged support (their plusses) along with other potentially telling factors like the partisan strength of their precinct or their partisan primary voting history and other significant demographic characteristics. These days the national parties are paying to develop modeling scores for their key races which take all of the available information about each voter and uses it to develop a number that measures the likelihood that the voter will support the candidate relative to all the other voters. They can use this info to determine which voters are very likely to support one candidate or the other, although it's a little bit harder to predict what happens with the 3rd party candidates.
Once you understand all of these variables it's clear to see why you can't just look at raw early/mail vote totals and draw conclusions from them. Here are the questions someone will need to ask of the campaigns:
Also, if you can get them to show their work that would be even better.