Which of the following statements is true? Check all that apply. We can use results from an observational study to test a claim about a population. We can use results from an observational study to establish an association between two variables. When an observational study has a large sample and follows people for many years, we can use the results to establish a cause-and-effect relationship between two variables. When many observational studies together meet specific criteria, the results can provide varying degrees of evidence for a cause-and-effect relationship between two variables, but we should be cautious in interpreting such results. A well-designed experiment is the only legitimate way to establish a cause-and-effect relationship between two variables.