Surveys most frequently study people at one point in time. On many occasions, however, researchers wish to make comparisons over time. For example, local newspapers often hire firms to conduct an annual random survey of county residents. Because the questions are the same each year, it is possible to track changes over time in such variables as satisfaction with the area, attitudes toward the school system, and perceived major problems facing the county. Similarly, a large number of first-year students are surveyed each year at colleges throughout the United States to study changes in the composition, attitudes, and aspirations of this group (Pryor, Eagan, Blake, Hurtado, Berdan, & Case (2012)). First-year college students today, for instance, come from more ethnically diverse backgrounds than those in the 1970s (90.9% of respondents in 1971 were White whereas in 2012, 69.7% were). Political attitudes have also shifted over time among this group: Trends in opinions about paying taxes and abortion rights can be seen. Finally, the percentage of new students who think that their “emotional health” is above average or in the “top 10%” is at a 25-year low in 2012: In 1985, 64% of respondents reported good emotional health; in 2012, 52% of students did.
Another way to study changes over time is to conduct a panel study in which the same people are surveyed at two or more points in time. In a two-wave panel study, people are surveyed at two points in time; in a three-wave panel study, three surveys are conducted; and so on. Panel studies are particularly important when the research question addresses the relationship between one variable at “time 1” and another variable at some later “time 2.” For example, Chandra et al. (2008) examined the relationship between exposure to sexual content on television and teen pregnancy over time. Data were collected from over 2,000 teens over a 3-year period. Exposure to sexual content on television was assessed using a survey that asked the participants to report on their television viewing habits, along with their sexual knowledge, attitudes, and behavior. Participants were surveyed three times over the course of 3 years. Chandra and her colleagues found that higher levels of exposure to sexual content on television were, indeed, predictive of higher rates of teen pregnancy—as shown in Figure 7.2. Indeed, they reported that “high rates of exposure corresponded to twice the rate of observed pregnancies seen with low rates of exposure” (p. 1052).
Probability of pregnancy at “time 3” related to exposure to low, medium, or high levels of sexual content on television at “time 1”
Adapted from “Does watching sex on television predict teen pregnancy? Findings from a national longitudinal survey of youth,” by A. Chandra, S. C. Martino, R. L. Collins, M. N. Elliott, S. H. Berry, D. E. Kanouse, and A. Miu, 2008, Pediatrics, 122, pp. 1047–1054.