Confounding in community interventions

Evaluations of community intervention projects usually use the community as the unit of analysis and employ a possibly costly cohort approach, which may not achieve sufficient power. The case-control approach, applied extensively in cancer control, may be an inexpensive alternative. Moreover, the case-control approach permits evaluation of potential confounding by factors possibly associated with the intervention. A simulation based on case-control methodology for calculating an intervention program effect (prevented fraction) demonstrates that the threat of confounding, at levels of prevented fractions in the range of 10% to 30%, is modest. Community-based interventions with lower prevented fractions are more susceptible, and their results should be considered with caution. Such boundaries may be of use to investigators in planning and evaluating community-based efforts to reduce risk and control disease.