One measure of the exposure of a person to tobacco smoke is the urinary cotinine-to-creatinine ratio (CCR). (Cotinine is formed in the body by breaking down nicotine.) Scientists measured this ratio in infants from smoking households (Blackburn et al. 2003). These households were divided according to their previous behavior into two groups: ones with strict controls to prevent exposure of the infant to smoke (31 babies) and another group with less strict controls (133 babies). The mean (and standard deviation) of the log-transformed CCR was 1.26 (1.58) in the babies from strict households and 2.58 (1.16) from babies from less strict households. The distribution of the log-transformed CCR was approximately normally distributed in both groups.

a. Do babies from households with strict controls differ significantly from those with less strict controls in their exposure to smoke? Perform an appropriate test.

b. On the non-transformed scale, how much higher is the CCR for babies in the less strict group, as compared to those in the strict group?

c. Is this an observational or experimental study? Explain.