How are TV ratings determined?

[Guest gusmahler]

I was under the impression that some people had boxes in their house that monitored what they watched and others used a notebook.  Now how does that work?  If you are switching between Raw and Monday Night Football, what happens to the rating?  Is it based on which show you watched for the majority of the quarter hour?  

 

If so, I guess that would explain ratings spikes: people switch to Raw, see Hogan and Rock and keep it there.  But they see Billy & Chuck, they switch back to Football.