A December 2023 poll conducted by the Economist ostensibly showed 20% of Americans under 30 agreeing with the statement that "the Holocaust is a myth." The Pew Research Center, a veteran polling outfit, argues that this number is wildly misleading, and that the actual number is probably closer to 3%.
According to Pew, the problem lies in online, opt-in polls which are increasingly popular. While they can be accurate, Pew shows that they tend to badly skew the views of those under 30, as well as Hispanic adults - not just regarding the Holocaust, but on a whole host of issues.
Pew argues the culprit is a group that deliberately opts in to give bogus or extreme answers for a variety of reasons, and which are unrepresentative of their cohort at large. Other polling methods control for this and minimize its impact.