The report's authors estimate that 601,027 Iraqis have suffered violent deaths since the March 2003 invasion. And the study suggests that the country has become more violent in the last year as sectarian violence has grown more intense.
"This clearly is a much higher number than many people have been thinking about," said Gilbert Burnham, the lead author and a professor at the Bloomberg School of Public Health at Johns Hopkins University. "It shows the violence has spread across the country."
Iraq's overall death rate has increased from 5.5 per 1,000 people a year, before the war, to 13.2 per 1,000 people a year after the invasion. The death rate spiked to 19.8 deaths for every 1,000 people between June 2005 and June 2006, according to the researchers, whose findings are being published this week in the Lancet, the British medical journal.
Most of the deaths reported in the study are of military-aged men. But the study's lead author said it was impossible to tell who among the people killed by violence were civilians, who were insurgents and who were members of the Iraqi security forces.
Burnham said he anticipated criticism of the study, which grapples with an issue that is sensitive to U.S. and British officials -- the undetermined number of war-related deaths. The same group of scientists conducted a study in 2004 that placed the number of deaths caused by the war in Iraq at about 100,000. That study was criticized on methodological grounds and British and American officials said it vastly overstated the number killed in Iraq.
The Pentagon reported in August that the number of civilian causalities had increased sharply, but did not provide an overall number.
On Tuesday, Pentagon officials did not comment on the study's numbers, and referred questions on civilian deaths to the Iraqi Ministry of Health. But Lt. Col. Mark Ballesderos, a Pentagon spokesman, said the U.S. takes care to avoid civilian casualties, while insurgents deliberately target them.
Some groups, like Human Rights Watch, had been skeptical of the last estimate done by the Johns Hopkins researchers. But Sarah Leah Whitson, the executive director of Human Rights Watch's Middle East division, said her group had no reason to question the accuracy of the new survey.
"If there is surprise about the size of the figure, it has more to do with our existing death tolls," Whitson said. "The conventional wisdom is based on shoddy information." But Michael O'Hanlon, a Brookings scholar who compiles civilian casualty estimates and who was critical of the last study, called the survey method flawed.
"The study is so far off they should not have published it. It is irresponsible," he said. "Their numbers are out of whack with every other estimate."
In 2005, the Bush administration has put the civilian casualty count at 30,000. A report by the Los Angeles Times in June, based on statistics from the Iraqi Health Ministry and the Baghdad morgue, estimated 50,000 civilians had been killed in the conflict. That is similar to the estimate by the British-based anti-war group Iraq Body Count, which puts the number of civilian casualties at between 43,850 and 48,693.
The Johns Hopkins estimate is based on a survey of 1,849 randomly selected households across Iraq. The results from that sample were then applied to the country at large, which has an estimated population of 25 million. The survey was conducted by physicians from Baghdad's Al Mustansiriya University School of Medicine. The Massachusetts Institute of Technology Center for International Studies was a sponsor of the study.
The Johns Hopkins researchers said methods that rely on morgue data or news reports may be inaccurate and undercount the deaths. Paul Bolton, an associate professor at the Boston University School of Public Health who reviewed the study at the behest of the authors, said that official statistics are often less reliable than surveys.
"Most of the time people would trust a random sample more than they would any kind of official reporting or official statistic," Bolton said. "Often we use this type of study to check if the official statistics are accurate."
Based on the survey results, the Johns Hopkins scientists are 95 percent certain that the number of Iraqis killed violently during the war ranges from 426,369 to 793,663. They said that 601,027 is the statistically most probable number.
"It is definitely an improved study" from the one done in 2004, Burnham said. "We learned from the experience last time. Last time, we did not expect to find so much violence. We expected to find deaths from diarrhea and environmental issues."
The percentage of deaths attributed to U.S. and coalition actions has been falling, but remains high. Between June 2005 and June 2006, 26 percent of the deaths were caused by coalition actions, 30 percent by other fighters, and 44 percent could not be determined. During the first year of the war, 36 percent of the deaths were attributed to the U.S. military operations.
The estimate, based on what Iraqi families told researchers, would suggest nearly 200,000 people were killed by American forces since the war began. But O'Hanlon said that asking Iraqis who is to blame for a family member's death would not yield an accurate response.
"You see very critical attitudes to the occupying power," O'Hanlon said. "People blame the U.S. for violent deaths that somebody else may have caused."
The study suggests the number of Iraqi dead is in line with civilian deaths in other conflicts including the Vietnam War, where it said an estimated 3 million civilians died; Congo, where 3.8 million have been killed; or East Timor, where 200,000 people, a quarter of the population, died.
"Now that we see the size of this conflict," Burnham said, "there needs to be serious discussion of how we lessen this impact, how we protect the populations better in future conflicts."
