There are at least four possible approaches to determining the number of people killed in a given instance of genocide or other crimes against humanity. The best estimate of such fatalities is possible if the perpetrators have kept accurate records, but this seems rare. A second approach requires the investigation of mass graves, either by taking an actual count of exhumed bodies, or making an estimate based on the number and the sizes of the graves. A third approach is the demographic analysis of census data or other population data. A fourth approach involves interviewing survivors of the violence about fatalities in their families, followed, in most cases, by a statistical extrapolation from the results of the interviews. The advantages and disadvantages of each approach can be understood by reference to the case of Cambodia, where all of these methods have been applied.

One of the first attempts to gauge the magnitude of the...