Chang is collecting data on the number of hours people sleep. Over a period of two weeks, he gathers information about the number of daily hours of sleep of 125 people at each of the following ages—15, 25, 35, 45, 55, 65, and 75 years—and searches for any apparent differences. Which type of developmental research design is Chang using?
a. correlational
b. cross-sectional
c. generational
d. longitudinal