The effectiveness of digital ads is wildly oversold. A large-scale study of ads on eBay found that brand search ad effectiveness was overestimated by up to 4,100%. A similar analysis of Facebook ads threw up a number of 4,000%. For all the data we have, it seems like companies still don’t have an answer to a question first posed by the famous 19th century retailer John Wanamaker: Which half of my company’s advertising budget is wasted?