The WHO estimate is based on interviews in more than 9,000 households across Iraq and is among the most comprehensive of such surveys to date.
The death toll in Iraq following the U.S.-led invasion in March 2003 has been a subject of much controversy. The U.S. military keeps a precise count of its own killed and wounded soldiers, but not of Iraqi military or civilian casualties.
Estimates of Iraqi losses range from a high of over 600,000 deaths, according to a study published in the British medical journal "The Lancet," to an estimate of some 48,000 deaths by 2006, according to the group Iraq Body Count, which tracks local press reports.
By the UN's own admission, the WHO survey does not purport to resolve the issue once and for all. The figure of 151,000 deaths between 2003 and 2006 is still an estimate, based on an extrapolation of data.
But the WHO study is among the best-organized and largest-scale efforts to arrive at an accurate number, according to co-author Ties Boerma.
"It is the best possible picture we could obtain with a survey of this size under these circumstances," Boerma says. "The true picture in exact numbers of deaths can only be revealed by a full and complete registration system, and that isn't present in Iraq right now and it wasn't present since 2003."
Boerma adds that "there are certainly ways to improve work in the future, in terms of death registration, in terms of hospitals."
Between 2006 and 2007, the WHO sent teams of surveyors to some 1,000 towns and villages across the country to talk to people in more than 9,000 households.
Importantly, the survey work was done by local experts, in full cooperation with local communities, according to Naima al-Gassir, the WHO's representative in Iraq.
"The communities were informed ahead of time, through different ways, either through the media or through letters that went through them or through community leaders, or the mosques, or the religious places or the schools, and that was also a strength because they would support the survey and allow the survey to go on -- also the community support to make a high response rate for the survey," al-Gassir says.
He said the interviewers were primarily physicians, nurses, pharmacists, and other health professionals from the ministries of Health and of Higher Education. Statisticians from the Planning Ministry provided technical support.
Al-Gassir highlights the "extreme difficulty" and risks that accompanied the researchers' work in preparing a "large, countrywide survey of households under such circumstances." He says there was acute attention paid to the protection of "interviewers, teams, and also those who were being interviewed," adding that one of the directors of the central statistics office was killed in Baghdad, an interviewer was kidnapped, and there were other close calls with violence.
According to the WHO, what is clear is that between 2003 and 2006, violence was a leading cause of death for all Iraqi adults and the single greatest cause of death in males aged 15 to 59 -- a reflection of the massive impact of the war on the entire population.
Full survey results have been published in the "New England Journal Of Medicine."
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