In each of the following settings, identify the explanatory and response variables. Then determine whether each is an experiment or an observational study. Explain your reasoning.

(a) 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. 272 Chapter 4 Collecting Data

(b) A recent study investigated the potential benefits of caffeine on athletic performance. Volunteers were randomly assigned to complete 10-km bike ride after consuming caffeine and others after consuming a placebo. The average time to complete the 10-km bike ride was 1.6% faster for subjects who were given caffeine than for subjects who were given a placebo.