The level of confidence is taught early in most statistics courses.
The only way to achieve 100% confidence is the 100% survey. Too costly but there are some that would do that (the census for example.)
How accurate are ratings surveys nowadays?
Fox is claiming number one in ratings on cable.
However how much of this is influenced by landline vs cellphone?
How much is influenced by people watching shows on computer and not cable or free to air via antenna television?
I think this is something to ponder especially for advertising agencies wanting bang for their buck.