Just about as long as marketing has been around, CEOs and marketers have debated how to measure success, which tools are the best and which proportion of marketing actually works.
Even with the arrival of digital media, marketers cannot yet measure outcomes with perfect certainty because of environmental and human variables. However, marketers are getting better at understanding ROI since digital marketing is more measurable than most forms of traditional marketing (aside from direct marketing or outgoing sales calls) ever were.