Actually you don't. You just think you do. I've made my living in advertising and marketing for the past couple decades. In advertising it was all about placing an ad at a place where the best audience - the most suitable persons - were likely to see it. That is what advertising does. We lived off the Nielsen ratings - those approximations of how many people of what age and socio-economic condition were watching a program at any given time. The margin of error was pretty big - 3.5% - so that a rating point of 1.0 (1% of the total audience) was +- 3.5% which meant that theoretically no one was watching or 4.5% were watching and the truth was somewhere in between. This was better known as a crap shoot.
In the middle 1990s someone in Jacksonville Florida started taking data directly off the cable box so that exact numbers could be extrapolated based on a 20,000 local sample. The industry went crazy over the debate centering on privacy v. precision. Fast forward that!
With Internet data collection (see Google) and the ability to match a household to Internet socio-graphic information, it is a simple set of steps to link up all the data and associate it with a person or persons. This is so far removed from Nielsen data of just 15 years ago as to make your head spin. If you own a Dish TV access system and are on the Internet, Google has all the information about you so that it could, if they have enough advertisers, show you only the ads that they know are relevant to you and exclude those which they know of that are of no interest.
The bottom line is that you have no privacy anymore as your data is sought after and of economic value. Don't kid yourself that all those people who collect it are 1. honorable and 2. discrete. Wiki-leaks should drive that point home.