Nobody knows what you're watching, unless you're one of the 5,000 Nielsen households equipped with a set top box that tracks/transmits this information (some families still use the old diary method).
In theory, cable companies *could* gather this information, but the current method is far cheaper and just as good given the sample size used (and the occasional audits done to make sure things are still lining up properly).
My last full-time job before retiring a few years back was Director of Technology with Nielsen Entertainment.
Since you retired, TV’s now report in realtime.
This would work if the sample did not know they were being sampled. Otherwise, if they are aware of how important they are to the networks, expect they will not behave normally. You could account for this by applying estimates but it is unmeasured in the pool. For example, I would expect them to cancel their service less than an unmonitored pool of similar composition.
Do they still use the set top boxes? I know of a current nielsen household and that family just wears a beeper looking thing all day that collects embedded information from TV and Radio at home and everywhere they go...