A case-control study compared 93 people who suffer from chronic migraine with aura (cases) to a sample of 93 healthy patients (control; Schwerzmann et al. 2005). The researchers used transesophageal echocardiography to look for cardiac shunts in all of patients. (A cardiac shunt is a heart defect that causes blood to flow from the right to the left in the heart, causing poor oxygenation). Forty four of migraine patients were found to have a cardiac shunt, while only 16 of people without migraine symptoms had this heart defect. (Whitlock, Schluter, 3rd edition) (a) Is this an observational or experimental study? (b) Find an estimate for the difference between the probability of cardiac shunt between people with and without migraine . Determine 95% confidence interval for this estimate. (c) How strong is the association between migraine and cardiac shunts? Cal- culate the odd ratio for migraine, comparing the patients with and without cardiac shunts. What is the 95%confidence interval this odd ratio? (d) Find an estimate for the relative risk of cardiac shunt between people with and without migraine and 95% confidence interval for it.