In the presentation we focused on brand keyword bidding. In this case, paid clicks potentially substitute organic traffic. The advertisement effect we observe is the total AdWords traffic which includes not only the truly gained traffic but also the clicks of individuals that would have visited the website even in absence of SEM. The latter is what economists call the missing counterfactual. Missing as we do not observe it. Without this data it is impossible to calculate the true ROI.
The way to find the missing counterfactual is through controlled random experiments. The presentation contains a complete AdWords optimisation toolbox in GNU R necessary to run controlled experiments.