Sleep and stress or stress and sleep? Consider the scenario described in Example 4.3. Make an argument for treating the Subjective Units of Distress Scale as the explanatory variable and the Pittsburgh Sleep Quality Index as the response variable.
EXAMPLE 4.4
Hemoglobin and anemia. Hemoglobin is a measure of iron in the blood. The units are grams of hemoglobin per deciliter of blood (g/dl). Normal values depend on age and gender. Adult women typically have values between 12 and 16 g/dl.
Anemia (lack of iron in the blood) is a major problem in developing countries, and many studies have been designed to evaluate the problem. In these studies, computing the mean hemoglobin level of a sample of people is not particularly useful. For studies like this, using a definition of severe anemia as a hemoglobin level of less than 8 g/dl is more appropriate. Now, researchers can administer different treatments and then compare the proportions of subjects who are severely anemic rather than compare the difference in the mean hemoglobin levels. The categorical variable, severely anemic or not, is much more useful than the quantitative variable, hemoglobin level.