4%.
Researchers have demonstrated repeatedly in human surveys the stylized fact that, far from being an oracule or gold standard, a certain small percentage of human responses will reliably be bullshit: “jokester” or “mischievous responders”, or more memorably, “lizardman constant” responders—respondents who give the wrong answer to simple questions.
Below a certain percentage of responses, for sufficiently rare responses, much or all of responding humans may be lying, lazy, crazy, or maliciously responding and the responses are false. This systematic error seriously undermines attempts to study rare beliefs such as conspiracy theories, and puts bounds on how accurate any single survey can hope to be.