The World Health Organization on Wednesday waded into the controversial subject of Iraqi civilian deaths, publishing a study that estimated that the number of deaths from the start of the war through June 2006 was at least twice as high as the oft-cited Iraq Body Count.
The study is the latest in a long series of attempts to come up with realistic numbers of civilian deaths. The numbers are politically fraught, and researchers' work has been further complicated by problems in collecting data while working in a war zone.
The estimates have varied widely. The Iraq Body Count, a nongovernmental group based in Britain that bases its numbers on news media accounts, put the number of civilians dead at 47,668 during the same period of time as the World Health Organization study, the W.H.O. report said. President Bush in the past used a number that was similar to one put forward at the time by the Iraq Body Count.
But another study, by Johns Hopkins, which has come under criticism for its methodology, cited an estimate of about 600,000 dead between the war's start, in March 2003, and July 2006.
The World Health Organization said its study, based on interviews with families, indicated with a 95 percent degree of statistical certainty that between 104,000 and 223,000 civilians had died. It based its estimate of 151,000 deaths on that range.
Those figures made violence the leading cause of adult male deaths in Iraq and one of the leading causes of death for the population as a whole, the health organization research team reported online in the New England Journal of Medicine. More than half the violent deaths occurred in Baghdad.
While the new study appears to have the broadest scope to date, increasing its reliability, well known limitations of such efforts in war areas make it unlikely to resolve debate about the extent of the killing in Iraq.
Iraqi officials gave conflicting assessments of the newest study, with one senior Health Ministry official praising it and another saying the numbers were exaggerated.
The White House said that it had not seen the study and would not comment on its estimated death toll, but that the recent increase in American forces had reduced civilian and military casualties. ''We mourn the deaths of all people in Iraq,'' said Jeanie Mamo, a White House spokeswoman.
In any case, the study ended four months after the bombing of a revered Shiite shrine in Samarra helped set off a wave of killings throughout Baghdad and other mixed Sunni-Shiite areas. So because of its timing, the study missed the period of what is believed to be the worst sectarian killings, during the latter half of 2006 and the first eight months of 2007.
The figures on violent deaths were part of a large study of chronic illnesses, mental health status, environmental risk factors and other factors affecting family health in Iraq. The figures were based on interviews with 9,345 heads of households across the country that had been selected according to statistical methods that are standard in peaceful areas. The interviewers, who were employees of the Iraqi Ministry of Health, had been trained how to ask the survey questions and to assign the stated causes of deaths.
The surveyors largely conducted their work in August and September 2006. In Baghdad, Shiite militiamen, often acting in coordination with or with the acquiescence of fellow Shiites in the Iraq security forces, purged many neighborhoods of Sunnis. Many were grabbed, handcuffed, shot in the head and dumped with other victims. Sunni insurgents continued their campaign of terrorizing Shiite areas with car bombs and other attacks.
In fact, one co-author, Louay Hakki Rasheed, was killed on his way to work on Aug. 2, 2007. The extraordinarily dangerous security situation prevented surveyors from visiting about 11 percent of the areas that the researchers had intended to visit.
Most of the places that were off-limits to the researchers were in Anbar Province, the Sunni-dominated region of western Iraq. While there have been significant security improvements in Anbar in the past year -- after Sunni tribal leaders joined with United States troops to drive out extremist militants -- in 2006 the province was a lawless haven dominated by insurgents.
Most of the other areas into which the researchers could not go for safety reasons were in Baghdad, which at the time was being ripped into balkanized concentrations of Shiites and Sunnis. Some neighborhoods looked like urban ghost towns, as the residents who did not have the money or the ability to flee the country stayed holed up in their homes rather than risk being abducted or killed by the death squads and gangs of criminals and insurgents who roamed much of the capital freely.
Iraqi authorities often have asserted that estimates of deaths provided by outside groups and researchers are too high. But there is a significant political element to the numbers, and as the surge in violent deaths in 2006 from death squad activities and other killings became a major embarrassment, the Iraqi government moved to sharply curb access to the data.
At the same time, Iraqi officials have asserted that they made improvements in their ability to track fatalities using morgue counts and other means. One shortcoming has always been that the corpses of many victims, if they are identifiable, are taken by family members straight to the cemetery, bypassing the morgue and hospital. Yet Iraqi authorities say that relatives still have an incentive to obtain a death certificate because it is required for inheritance, for government compensation, and for other purposes.
In a telephone news conference organized by the health organization, a voice identified as that of the Iraqi health minister, Salih Mahdi Mutlab al-Hasnawi, said, ''It is a very sound survey, and the sample is a good sample,'' and ''I believe in those numbers.''
But a senior official in the Iraq Health Ministry's inspector general's office cast doubt on the findings, saying 151,000 was far too high. The official, who said he was not allowed to speak about the matter and refused to allow his name to be used, said the numbers cited by the study were much larger than figures tracked by the ministry. But he refused to provide any alternative tallies for the death toll, saying he was not authorized to do so.
Mohamed M. Ali, a health agency statistician and co-author of the report, said that ''in the absence of comprehensive death registration and hospital reporting, household surveys are the best we can do.'' Even then, the figures collected are likely to be underestimates because ''some homes could not be visited because of high levels of insecurity and more people move residence in times of conflict,'' Mr. Hasnawi, the health minister, said in a statement issued by the W.H.O.
To come up with estimates for the 11 percent of target areas they could not reach, the researchers used a formula that was based primarily on the Iraq Body Count to determine how much higher the number of deaths could have been there than in other areas of the country.
The Iraq Body Count project bases its numbers on news media reports. That count registered 47,668 civilian deaths because of violence in the study period, a figure that the health organization considered low because many such deaths are not reported in the news media.
The Johns Hopkins study, which was published in The Lancet in October 2006, estimated that 601,027 Iraqi civilians had died from violence. That study, which was conducted with researchers from Al-Mustansiriya University in Baghdad, involved one-fifth the number of households and one-twentieth the number of areas surveyed by the new W.H.O. study.