But Scott Zeger, the head of the department of biostatistics at Johns Hopkins, who performed the statistical analysis in the study, points out that clustered sampling is the rule rather than the exception in public-health studies, and that the patterns of deaths caused by epidemics are also very variable by location.

ECONOMIST: The Iraqi war

Dr. Pedersen, a professor in the department of medical epidemiology and biostatistics at Karolinska Institute, currently studies dementia among twins, including the recent research showing that an identical sibling of a twin with dementia has an increased risk of also developing the condition.

WSJ: Twins Health Registry Is Treasure Trove for Medical Research