The companies operate on commercial basis, deriving most of their revenue from the sale of advertising tome. The financial resources, advertising revenue and programme production of the companies vary considerably, depending largely on the size of population in the areas in which they operate. Although newspapers may acquire an interest in programme companies, there are safeguards to ensure against concentration of media ownership, thereby protecting the public interest. Each programme company plans the content of the programmes to be broadcast in its area. These are produced by the company itself, or by other programme companies or bought from elsewhere. The five largest companies – two serving London and three serving north-west England, the Midlands and Yorkshire – supply more programmes for brascast elsewhere on the national network than do the smaller ones. A common news service is provided 24 hours a day by Independent Television News (ITN).