observational study
study that draws inferences from a sample to a population, in cases where the independent variable is not under the control of the researcher because of ethical concerns or logistical constraints
case series
type of medical research study
Total diet study
assessment of consumers' average dietary intake of contaminants and nutrients
case-control study
type of observational study comparing two existing groups differing in outcome
time and motion study
business efficiency technique
natural experiment
empirical study in which individuals/clusters exposed to the experimental and control conditions are determined by factors outside the control of the investigators (nature), but the process governing the exposures arguably resembles random assignment