Which determines whether groups of people that have been exposed to a chemical have experienced a statistically greater degree of harm as opposed to a group that was not exposed?

Epidemiological Studies

Explanation:

Epidemiology is the analysis of infections in humans or other wildlife populations, primarily how, where and when they take place.

Epidemiologists try to identify which variables are correlated with disease (risk factors), and which factors can protect people or animals from disease (protective factors).

Such kinds of research monitor populations over a long period of time to demonstrate the risk of harm from exposure.