gains are taking place. The Cartagena privatization achieved significant improvements in efficiency. This is reflected in the doubling over a decade (1995–2005) in the ratio of collected revenues to operational costs. At the same time, the average tariff was halved in real terms, suggesting that a significant portion of the savings achieved through efficiency gains was passed to customers. Other impacts Water rationing was less frequent in ten cities five years after the private sector became involved in providing services. Studies based on household and public health surveys show that private operators tend to achieve better potability figures than public water utilities. Local operators The largest private water company in Colombia is Triple A (AAA), a Colombian company serving more than 1.5 million people. It has a strategic partnership with the publicly owned water utility of Madrid (Canal Isabel II). It is in practice managed by nationals and has been branding itself as a Colombian private company. The company was willing to focus on a market segment that other companies tend to avoid: the poorest cities. For example, in Barranquilla, more than 76 percent of AAA's customers are in the three lowest of six income segments used by the Colombian government. In Soledad, 98 percent of the population is in the three lowest segments, and poverty levels are similarly high in all the other municipalities where AAA operates. Other Colombian water operators are EIS, Conhydra and Sala. Colombian construction companies that were active in the water sector were awarded PPP contracts following tenders in which only local investors participated. In all these cases, governments chose to ease prequalification criteria to increase competition, resorting to various mechanisms to ensure the winning bidder would be able to operate the water utility. In Colombia, the winning bidders contracted experienced technical staff (often former managers and engineers from public utilities). Between 2001 and 2004, Colombian investors won almost all the PPP contracts awarded in that country under the PME program. Example of Barranquilla In 1996, more than 60 percent of Barranquilla's population had no water service at all or had it for only a few hours a day. The private operator was initially met with skepticism. When AAA began to install residential water meters in homes that had never had them, many consumers worried that the devices would result in big water bill increases. Opposition politicians like Guillermo Hoenisgberg initially opposed the private operator