How are television ratings calculated?

Television ratings are calculated by several researching firms. Video Research Ltd. is one of the more reliable firms, and more television networks, advertisers and Japanese drama fans use the numbers from this firm than any other. The ratings focus on the Kanto (Tokyo) and the Kansai (Osaka) areas, which are believed to serve as a good representation of what most of Japan watches. The ratings become available for the general public every Wednesday.