I learnt an old adage while doing my MBA: "Half the money I spend on advertising is wasted; the trouble is I don't know which half." - attributed to John Wanama

I learnt an old adage while doing my MBA: "Half the money I spend on advertising is wasted; the trouble is I don't know which half." - attributed to John Wanamaker. Online advertising was supposed to fix this. You could never tell when a potential customer read a billboard, but you could absolutely determine when someone clicked on an ad. I don't know what happened. I suspect everyone got obsessed by tracking user behaviour and remarketing and all the bells and whistles.