According to the site, TIOBE index is "not about the ''best'' programming language or the language in which ''most lines of code'' have been written". However the site does claim that the number of web pages may reflect the number of skilled engineers, courses and jobs worldwide.
The index is calculated from the number of search engine results for queries containing the name of the language. The index covers searches in Google, Google Blogs, MSN, Yahoo!, Baidu, Wikipedia and YouTube.
The index is updated once a month. The current information is free but the long term statistics over many years of observation is for sale. The index authors think that it may be valuable when accepting various strategic decisions.