What is an observational study?
A.
An observational study measures the value of the response variable without attempting to influence the value of either the response or explanatory variables.
B.
An observational study is a list of all individuals in a population along with certain characteristics of each individual.
C.
An observational study is when a researcher assigns individuals to a certain group, intentionally changing the value of an explanatory variable, and then recording the value of the response variable for each group.