There was an opt-in survey in from YouGov & The Economist that seemed to show a fifth of young Americans denied the Holocaust. The Economist ran a piece on it.
Well, Pew Research did a random sampling and found completely different and far less worrying results.
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This is very important to realise. For instance in South Africa, opt in opinion polling done by the Cape Independence Party and Victory Research shows that the majority of the people of Western Cape support Cape Independence (you can see this on their highly misleiding and obviously curated Wikipedia page), yet in reality the Cape Independence Party failed to get enough signatures to even appear on the ballot. They got 200 of the 7000 signatures required. The Cape Independence Party claimed previously to have over 80 000 signatures, yet there is no proof of these signatures anywhere.