http://www.economist.com/news/finance-and-economics/21581715-simple-tests-can-overstate-impact-search-engine-advertising-ad-scientists
Establishing cause and effect in offline advertising is hard. Ads are difficult to target: space on billboards and in newspapers is seen by lots of shoppers. Some of these eyeballs are worth spending money on; others, either because they belong to existing customers or to people who never will be, are not. And even when big ad campaigns are followed by strong sales, the intuitive conclusion—that rising sales are the result of good ads—can be misleading. Advertising budgets often rise in good times so that spending and sales grow together, even if the advertisements are useless. The ads and the sales have a common cause—strong demand—but may have no causal link.
