The study is really broken from the beginning: ask someone who take drugs to assess their own capacities, instead of measuring those from an objective perspect

The study is really broken from the beginning: ask someone who take drugs to assess their own capacities, instead of measuring those from an objective perspective. Obviously, "yes cannabis affects your ability to self-report your cognitive abilities"; it's a psychoactive drug that is mainly used for its cognitive effects, it's the main use/"advantage" of it ! (more rarely for pain management) It's like if you ask alkaloidal cocaine (crack) users how they feel about their abilities. They may report amazing bodily control enhancement, and seen from the outside it may be very different. Even alcohol users, at the exit of night clubs, you sometimes see them report self-report increased abilities and decide to drive. In that case, asking them to self-report doesn't give you much info on their actual abilities.