You take a math test before you get interviewed. if you do not pass the math test they will not bring you back for a interview. the math test is very short and they give you a hour to do it.
I applied through an employee referral. The process took 2 months. I interviewed at Harmelin Media (Philadelphia, PA) in Apr 2013
Interview
Before the interview, each person is required to take a math test. Then you interview with HR, if there is time that day you could possibly interview with the account team you would potentially working with.