Radioactive twins. (Further details of this study are given in Chapter 4 on page 282.) Most people believe that country air is better to breathe than city air, but how would you prove it? You might start by choosing a response that narrows down what you mean by “better.†One feature of healthy lungs is that they are quick to get rid of any nasty stuff they breathe in, such as particles of dust and smoke. This study used as its response variable the rate of tracheobronchial clearance, that is, how quickly the lungs got rid of nasty stuff.
Investigators managed to find seven pairs of identical twins who satisfied two requirements: (1) One twin from each pair lived in the country and the other lived in a city, and (2) both twins in the pair were willing to inhale an aerosol of radioactive Teflon particles. That was how the investigators measured tracheobronchial clearance. The level of radioactivity was measured twice for each twin—right an inhaling and then again, an hour later. The response was the percentage of original radioactivity still remaining 1 hour after inhaling. The data are shown in Display 9.85.
a. Discuss whether a statistical test is appropriate.
b. Regardless of your answer to part a, carry out the test.
c. Tell how to design a study that uses two independent samples (as an alternative to paired samples) to compare tracheobronchial clearance rates for people living in urban and rural environments.
 d. What are the advantages of using paired data from a single sample?
e. What are the advantages of using two independent samples?
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