For example, interviews in displacement camps must be used as substitutes for body counts and population-based census data.
Surveys also vary in coverage, and years of war and famine throughout Sudan have "reconfigured nuclear families, making sampling units in surveys problematic," the authors write. (Related feature: "Shattered Sudan" in National Geographic magazine.)
The study also says that estimates of Darfur mortality "have been based on the dubious assumption of a constant number of deaths per month."
According to Hagan and Palloni, the State Department's data were limited due to an emphasis on "camp health problems rather than pre-camp violence."
The department also drew "on health surveys that were not fully identified and for which primary sources are uncertain."
State Department officials declined to comment for this article.
To reach their estimate, Hagan and Palloni derived direct crude mortality rates, or CMRs, from survey materials recorded by the Sudanese state of West Darfur. They also made indirect CMR estimates using child mortality rates.
The pair believes that the direct estimates are too high while the indirect estimates are too low, so they combined the data sets to produce upper and lower bounds.
They then calculated the number of deaths using the CMRs and estimates of the affected populations.
"Our conclusion is that the total number of deaths is 200,000 or more, possibly much more," Hagan said.
Before the State Department's 2005 release, the World Health Organization issued a 2004 report estimating that 70,000 people had perished over a seven-month period because of the Darfur conflict.
By April 2005 a United Nations humanitarian coordinator had estimated 180,000 people had perished over 18 months, and UN Secretary General Kofi Annan had put the number at 300,000.
Eric Reeves, an influential Sudan activist and professor at Smith College in Northampton, Massachusetts, says that the U.S. government's low estimates were politically tainted.
"The State Department has not wanted Darfur to be perceived as the enormous genocidal crisis that it is, for lack of an effective way of responding," he said.
"This is propaganda, not epidemiology [the study of patterns and causes of diseases, injuries, and other human health problems]."
Reeves, who puts the number of deaths at 500,000, gives particular prominence to a well-known study that focused specifically on lives lost to direct violence.
That study, conducted in 2005 by the Coalition for International Justice (CIJ), a now-defunct legal nonprofit, was not used in the new report.
Reeves said it was "irresponsible" of the researchers to exclude the CIJ research, noting that study co-author Hagan used it last year to conclude that at least 390,000 Africans had perished in the conflict, a figure higher than his current estimate.
Hagan says that the CIJ survey was a rich resource that simply did not fit the criteria for the current study.
He said the new estimate "was intended to establish a floor level of 200,000 deaths, so that media sources would no longer underestimate the urgency of the situation in Darfur."
He stressed that the actual number of deaths could be much higher.
The new estimate, he says, is more conservative than the previous one because it did not include missing persons who were presumed dead, and it used different criteria to measure family membership.
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