How could the ones who didn't vote vote for Trump.
Millennials aren't a real thing.
You might be misunderstanding what Zowie is saying.
He's asking if the statistic which is the topic of this thread has had it's base rate applied.
For instance suppose 40% of millennial's voted for Trump, but only 50% of millennial's voted. Applying the base rate of 50% would reduce the number of millennial's who voted for Trump by half down to 20%. Since the thread title says "Forty percent of white millennials voted for Trump" but the adjusted statistic is actually only 20% the basis of the thread topic would be invalid.
It is a very common thing to have statistical numbers which have not had the basis applied to be used to make a sweeping point, and when statistics are used in that way it's a formal fallacy of logic. It's always wrong.
Here's an example from Wikipedia on this issue of base rate neglect:
https://en.wikipedia.org/wiki/Base_rate_fallacyDrunk driversA group of police officers have breathalyzers displaying false drunkenness in 5% of the cases in which the driver is sober. However, the breathalyzers never fail to detect a truly drunk person. One in a thousand drivers is driving drunk. Suppose the police officers then stop a driver at random, and force the driver to take a breathalyzer test. It indicates that the driver is drunk. We assume you don't know anything else about him or her.
How high is the probability he or she really is drunk?
Many would answer as high as 95%, but the correct probability is about 2%.
An explanation for this is as follows: On average, for every 1,000 drivers tested, 1 driver is drunk, and it is 100% certain that for that driver there is a true positive test result, so there is 1 true positive test result.
999 drivers are not drunk, and among those drivers there are 5% false positive test results, so there are 49.95 false positive test results.
Therefore, the probability that one of the drivers among the 1 + 49.95 = 50.95 positive test results really is drunk is 1/50.95 = 0.019627.