Beyond the ecological fallacy: potential problems when studying healthcare organisations.

Ecological studies, which consider patient groups rather than individuals, are common in health policy research. The ‘ecological fallacy’ is a well-recognised methodological concern, but in this perspectives paper, we focus on less often appreciated but equally important limitations of such studies. In particular, we consider reliability and power as they apply to ecological studies, and make recommendations to inform the appropriate design and interpretation of these increasingly popular studies.