A researcher evaluates a new cholesterol medication by measuring cholesterol levels for a group of patients before they begin taking the medication and after they have been taking the medication for eight weeks. A second researcher measures quality of life for a group of 40-year-old men who have been married for at least 5 years and a group of 40-year-old men who are still single. Explain why the first researcher is probably not concerned about the validity of measurement, whereas the second researcher probably is.