Even when good census data are available, as in California, other factors such as the proportion of people commuting to and from the affected area may greatly affect the size of the population present at the actual time of the earthquake. Thus, even estimating the population at risk may be difficult, let alone selecting appropriate control subjects (133). As a result, almost all of the published epidemiologic studies on earthquake-related injuries are descriptive rather than the analytic studies necessary to test hypotheses that particular types of exposures or hazards (e.g., collapsing buildings) are associated with injuries.

(See chapter on Practical Application of Epidemiologic Methods for Disasters [Chapter 2], for a more in-depth discussion of the problems of conducting studies following disasters [e.g., collecting reliable data on injuries under difficult field conditions, collecting "perishable" data, dealing with a lack of definition of disaster-related injury, working with poor documentation in medical records, and selecting appropriate control subjects]).