A recent Harvard study concluded that people who drink coffee tend to live longer than those who don’t drink coffee. The study followed more than 20,000 nurses and doctors over a 30-year period, giving them frequent questionnaires to answer about their coffee habits. Researchers found that people who drank 3 to 5 cups of coffee a day had a mortality rate during the study that is 15% lower than those who did not drink coffee. Identify the explanatory and response variables. Then determine whether each is an experiment or an observational study. Explain your reasoning.