By watching what you're watching they can make guesses as to your race, sex, income, politics, marital status, how old your children are, what your interests ar

By watching what you're watching they can make guesses as to your race, sex, income, politics, marital status, how old your children are, what your interests are, etc. Not all of that will be 100% accurate, but they don't care. Most houses have more than one screen. It shouldn't be too hard to correlate what everyone is watching or playing at any given time, and this would give quite accurate data.