Which is affected by about a billion other metrics, like if it's sunny out. Attribution is the fundamental problem of advertising, and getting closer to solving it is Google and Facebook's value proposition. This indicates that they are not nearly as helpful in solving it as they have been claiming since they've been providing bad inputs for their customers to do analysis.

That works for impulse buys online I expect; doesn't seem useful beyond that. If you're running a one day festival you can have fantastic ad response - but if it rains the visitor figures are rubbish.

That's when you rely on metrics like bounce/view period to decide if your advertising worked.