I'd also like to know this. Do they count the people that tune in for only a part of the game? For example, I know I usually couldn't make it past the 3rd inning this year. I think they're measured by having some households a device that tracks their TV viewing habits. This is then extrapolated to the entire region. I don't know how you could do that though. You just don't take 1 or 5% of the city, take the percentage of those who switch on the game, and then extrapolate from that, to 100%. It's just not going to be accurate. I always thought it worked like radio in that they could simply detect the signals coming in and when they switched off. Statistically speaking, it is very accurate with of course the margin of error added in. People use sample sizes of 1-5% (even less) in many important estimations, including government and business statistics.