But clients don't (want to) belive you.
Then some consultant comes in with a econometric model built with half a data set, lots of dummy variables or worse than that neural networks completely confusing cause and effect. The outrage.
Anyway nice when someone like Stephen Levitt (Freakonomics) says it is difficult, and it is on film, and it is funny so you can show it to someone without hammering the point home.
If you don't want to watch it all, watch the 5 minutes 15 minutes in.