You use promotional codes for attribution, then you have complete knowledge. So it's extremely easy, and a solved problem. No need for analytics, no need to infringe on privacy.
I watch a youtube video and the youtuber shills me some product along with a promo code. A week later I decide to buy the product, but I don't remember what video had the ad or what the promo code was so I just don't bother with it. How are you going to attribute this sale to the youtuber?
That quote's from around 1900 but your point stands for print. Nielsen actually got really good numbers for TV using statistics, self-filled form sheets and crisp one-dollar bills.
