Answer:
observational study
Step-by-step explanation:
an observational study is a sample survey of a population whereby a sample of the population is used to make inference and the independent variable is out of the control of the researcher. Unlike the case of randomized experiments where subjects could be assigned randomly to groups, the researcher has no control over variables as he draws sample directly from population. There are three types of observational study:cross-sectional studies, case-control studies, and cohort studies.