Far, far more important than the size of the sample, however, is the degree to which the overall sample is truly random, that is, trule representative of the population sampled and here is where the first of many serious questions about the NEJM effort arise. As the authors themselves admit, they did not visit a significant proportion of the original designated clusters: "Of the 1086 originally selected clusters, 115 (10.6%) were not visited because of problems with security," meaning they were inconveniently situated in Anbar province, Baghdad, and two other areas that were dangerous to visit, (especially for Iraqi government employees from a Shia-controlled ministry.) While such reluctance is understandable--one of those involved was indeed killed during the survey--it also meant that areas with very high death tolls were excluded from the survey.
To fill the gap, the surveyors reached for the numbers advanced by the Iraqi Body Count, (IBC) a U.K. based entity that relies entirely on newspaper reports of Iraqi deaths to compile their figures. Due to IBC's policy of posting minimum and maximum figures, currently standing at 80,419 and 87,834, their numbers carry a misleading air of scientific precision. As the group itself readily concedes, the estimate must be incomplete, since it omits deaths that do not make it into the papers, a number that is likely to be high in a society as violently chaotic as today's Baghdad, and higher still outside Baghdad where it is even harder for journalists to operate.
Nevertheless, the NEJM study happily adopted a formula in which they compared the ratio between their figures from a province they did visit to the IBC number for that province, and then used that ratio to adjust their own figures for places they did not dare go. Interestingly, the last line of the table on page 8 of the Supplementary Appendix to the report, "adjustment for missing clusters using IBC data," reveals that in using the Body Count's dubious figures to fill the holes in their Baghdad data, the formula they employ actually revises downward the rate of violent deaths on what they label "low mortality provinces."