Obviously this method has every limited application in most instances, because of the impossibility or impracticability of segregating specific items as coming from separate purchases.
Doctors and administrators have long argued that tracking patients after treatment would be too difficult and costly, and unfair to providers lumbered with particularly unhealthy patients.
Veganism is on the rise, but Getting 100% of Americans to go plant based is unrealistic, so let's be optimistic and say we got 10% of the United States to stop eating meat.