Marketers worldwide grew up with John Wanamaker’s famous marketing aphorism, “Half of my advertising is wasted; I just don’t know which half.” That pithy quote is now a misleading anachronism; we can know which half. Digital media and tracking technologies, along with dramatically improved analytics, now mean that serious marketers — if they really care to know — can have an excellent idea of just how effective (or wasteful) their advertising really is. Ignorance is now a choice, not an excuse.