Digital ads are getting smarter. Are advertisers?

Check out a pair of shoes online, and you may find them following you around: in a display advertisement parked in the margin of a news story, or in the otherwise minimalist corner of a search-engine results page, or perhaps in the subject line of an email.

Why companies may be overpaying for web search ads

Paid internet search ads, also known as search-engine marketing (SEM), dominate online advertising. According to the latest figures available, advertisers spent $18.4 billion on paid search ads in 2013, or 43 percent of total US internet advertising revenue.