Polling by YouGov made headlines around the world when it suggested 20% of young adults in the US thought the holocaust was a myth.

But polling experts at the Pew Research Centre thought the result might not be accurate, due to problems with the kind of opt-in polling it was based on. They tried to replicate the finding, and did not get the same answer.

We speak to Andrew Mercer from the Pew Research Centre and YouGov chief scientist Douglas Rivers.

Presenter /series producer: Tom Colls