Media producers are very keen to know how many people are watching, buying and enjoying their products. Every week the list of the top ten grossing films in the country is published in many newspapers and magazines; bookshops display the best-selling books, often divided up into various categories such as fiction and biography; BBC Radio 1 devotes much of Sunday afternoon to counting down the top forty. Being at ‘number one’ in any chart is a big accolade, not only because it means lots of people like your product, but also because the ensuing publicity is a way of keeping that product in the public eye. Measuring audiences in this way – by adding up the number of people who consumed a product – seems obvious and straightforward, and the simplest way of knowing how audiences behave.