Which of the following statements is NOT true?
1) A well-designed experiment is the only legitimate way to establish a cause-and-effect relationship between two variables.
2) We can use results from an observational study to test a claim about a population. 3) 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.
4) 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.