Brands are calling out for clear ROI from ad campaigns – how can the industry deliver?

In an interview this week a frustrated sounding Conrad Bird, the marketer responsible for communications spend at Number 10 Downing Street, said ‘I need ROI not eyeballs to show my ad is effective’. 
In his case, where every pound of taxpayers money spent on advertising means a pound less to spend on other public projects, proving ROI is an absolute essential, but marketers across all verticals face the same challenge. How to prove that spending a sum of money will deliver the b