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158 PART II: Descriptive MethodsKey ConceptLongitudinal Design• In the longitudinal design, the same respondents are surveyed over time inorder to examine changes in individual respondents.• Because of the correlational nature of survey data, it is difficult to identifythe causes of individuals’ changes over time.• As people drop out of the study over time (attrition), the final sample mayno longer be comparable to the original sample or represent the population.The distinguishing characteristic of the longitudinal design is that the samesample of respondents is surveyed more than once. The longitudinal design hastwo important advantages. First, the investigator can determine the direction andextent of change for individual respondents. Also, because changes in each individual’sresponses are assessed, it’s easier to investigate reasons for attitude orbehavior changes. Second, the longitudinal design is the best survey design whena researcher wishes to assess the effect of some naturally occurring event.For example, Lucas (2005) examined changes in life satisfaction before andafter divorce in an 18-year longitudinal study of German households that beganin 1984. Many cross-sectional surveys have demonstrated that divorced peopleare less satisfied with life than are married people. Lucas sought to determine ifdivorce causes lower life satisfaction. Results indicated that these individuals’life satisfaction dropped before the divorce and gradually increased again followingthe divorce but did not return to their baseline state, indicating thatthe divorce likely decreased life satisfaction. However, Lucas also discoveredthat people who eventually divorced were less satisfied at the beginning of thestudy than those who stayed married—even before either group was married.Lucas concluded that the relationship between divorce and life satisfactionis due to preexisting differences in life satisfaction and to lasting changes dueto divorce.Heatherton, Keel, and their colleagues have used the longitudinal designto investigate changes in attitudes and behaviors related to eating duringthe transitions from college to early adulthood and from early adulthood tomiddle years (Heatherton, Mahamedi, Striepe, Field, & Keel, 1997; Keel, Baxter,Heatherton, & Joiner, 2007). Although much is known about eating disorders inadolescents and college students, less information is available about how disorderedeating may progress as individuals settle down, marry, establish careers,raise children, and gain a stronger sense of identity. These researchers hypothesizedthat as individuals change their roles and life goals during adulthood,their emphasis on physical appearance may decrease, which would decreasethe prevalence of eating disordered attitudes and behaviors (see Figure 5.4).The first “panel” of the study took place in 1982, when a randomly selectedsample of 800 women and 400 men from a private northeastern college wasasked to complete a survey about eating and dieting. The response rate was 78%(N 625) for women and 69% (N 276) for men. In 1992 the researchers contactedthese same individuals (with the help of the alumni office) and gave themthe same survey again about their eating attitudes and behaviors. The thirdpanel of data was collected in 2002, when the same individuals were in theirearly forties. The distinguishing characteristic of the longitudinal design is