The NGO US-El Salvador Sister Cities takes a look at how access to potable water is measured in an article titled What the Census Didn't Count: Water Rights and Privatization in El Salvador. Here is an excerpt:
[W]ith last month’s release of the 2007 National Census, the Salvadoran Government has tried to paint a brighter picture, fueling the ongoing controversy. ANDA recently claimed that the Census results prove that 8 out of 10 Salvadoran families now have access to potable water. However, according to the CDC [Center for the Defense of the Consumer], who cite United Nations Development Program numbers, that claim is deceptive.
In order to claim 80 percent water access, ANDA must count not only the sixty percent of the population that has access to potable water within their house or on their property, but also include another twenty percent of the population that must access water from public sources, adjacent private sources, or nearby wells. Especially in rural areas which have been historically marginalized by the central government, only 33.7 percent of the population has water access on their property, leaving a majority without secure access to water.
Furthermore, the CDC questions the indicators used to qualify sources of potable water, since water sources are not uniformly subject to quality standards. Water distribution is another concern, since it is not included in criteria for classifying homes as having access to water. While homes may be connected to a water distribution system, there may only be water in their pipes periodically at best.
Likewise, according to the CDC and the Center for Investigation of Salvadoran Public Opinion, 5 in 10 Salvadorans state they are subject to water rationing and the water they receive is not fit to drink. Also, 90 percent report water shortages at some time. This raises concerns as to the real progress being made towards improving water quality and access in the population as a whole, since building water infrastructure and piping to communities doesn’t immediately translate into quality potable water access.
According to the UNDP numbers, after taking all these factors into consideration, nearly 60 percent of households in El Salvador do not have indoor potable running water connections. According to the CDC, even those who do have running water often opt to buy bottled water if they can afford it, because of poor quality.