Walk outside your home, and you’re bound to see ads everywhere. On billboards, posters, buses, and even subway platforms. You’ve probably bought something after seeing these ads (as have millions of other people).  The reason? These ads are effective. They tug at your heartstrings, make you miss something you never had, or convince you that your life will be better once you sign up for that gym membership.  But how do brands know these ads will work? By testing...