To avoid undesirable content in the search indexes, webmasters can instruct spiders not to crawl certain files or directories through the standard robots.txt file in the root directory. Search engines are not paid for organic search traffic, their algorithms change, and there are no guarantees. In most cases, when Google is not leading in a given market, it is lagging. A successful Internet marketing campaign may also depend upon building high quality web pages to engage and persuade, setting up analytics programs. The purpose of such a network is usually to promote the other blogs in the same network and therefore increase the advertising.