By the very nature of their field of study, child development researchers are concerned with change that occurs over time. This fact brings to light another research design choice that must be considered: longitudinal research versus cross-sectional research. Longitudinal studies involve studying the same group of participants over a particular time period. Cross-sectional studies involved studying groups of participants in different age groups at the same point in time. It would seem that longitudinal research would be the developmental researcher's first choice, but because of some of the disadvantages of that method, developmental researchers must sometimes use the cross-sectional method.
In a longitudinal study, a researcher performs repeated observations or testing at specified points during the participants' lives, thus allowing the observation of development. The time span involved may be anywhere from a few months to a lifetime. This design provides the best information about the continuity or discontinuity of behavior over time and allows for the individual tracking of patterns of behavior, as well as trends of development, within a similar group. The problems of this type of design can often override the benefits. It is expensive to study a large group of individuals over an extensive time span. Keeping up with these individuals can be costly and time consuming. Participants may move away from home and local communities, and therefore drop out of the study. Sometimes the participants become wise to the testing or observations and practice over repeated measures and contaminate the results. Cohort effects may also be a factor in the outcomes of longitudinal studies. Cohort effects are common characteristics or trends in development for one cohort or group that may not follow suit for another cohort. To try to alleviate some of these potential problems, researchers often study small groups of individuals; but this may make it difficult to generalize the findings to a larger group.
Cross-sectional studies are quick by nature in that a researcher does not have to follow the development of each individual. At the same time a researcher does not gain the rich data on individual development that can be garnered from longitudinal studies, since the evidence of change is inferred from differences between the age groups. The cross-sectional design is also affected by the cohort effect where age differences may show trends particular to a specific group and not true developmental changes. Thus, while the cross-sectional design solves some the problems associated with a longitudinal design (e.g., subject dropout and cost), the cross-sectional design still suffers some disadvantages (e.g., age differences do not show age change and cohort effects).
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