What limitation prevents Hilda from concluding that a natural disaster caused depression among its victims?

The correct answer highlights that lacking a comparison group prevents Hilda from drawing a causal conclusion about the relationship between a natural disaster and depression among its victims. A comparison group is essential in research because it allows for the analysis of differences between those affected by the disaster and those who were not affected. This comparison helps to control for confounding variables and alternative explanations that could account for changes in depression levels. Without this group, Hilda cannot ascertain whether the observed increase in depression is directly attributable to the disaster itself, or if it might be influenced by other factors.

For instance, if she only examines victims of the disaster, she might overlook the possibility that their depression could be due to pre-existing conditions, personal history, or other environmental stressors. Establishing a thorough comparison enables researchers to better interpret the data and improve the validity of their conclusions regarding causation.