From the scientific method which overcame personal prejudice and relied on investigation under controlled conditions. These marvellous organisations, for our benefit, do this for us, and the links I provided, that you are so reluctant to read, are from their thousands of hours of toil.
This is especially important in this area where addicts are prone to lying. And where people can have considerably different experiences. And the reason I know these latter two points is again because of the scientific method.