One method, to get a rough estimate, is by judging electicity surges, i.e at the end of a programme, people putting kettles, etc, on. Think an organisation with the acronym BARB ( don't know what it stands for ) uses some method of places boxes placed on top of TV's to record what channel is being watched, at what time, and by what member of a selected family. Family selected if they are "average" family. This was method was used a few years ago, but technology has moved on somewhat, so this method may be obsolite