Longitudinal and cross-sectional designs are described as developmental research strategies. According to Berk, a longitudinal developmental research design relates tothe same groups of participants continuously at different ages. Oppositely, a cross-sectional developmental research design investigates groups of participants differing in age at the same time. Therefore, the difference between research designs is that longitudinal research design allows for studying patterns and differences of the same participants in early and later events. In contrast, the cross-sectional research design is more effective and easierto implement due to its flexibility and independence from participant dropout. In the longitudinal design, “the time spanned may be relatively short (a few months to several years) or very long (a decade or even a lifetime).”
The cohort effect can be described by the influence of a specific set of historical and cultural conditions on people born in the same period. Therefore, the findings and results solely based on one cohort do not apply to people developing at different times. In longitudinal studies, the example of cohort effects includes particular experiences affecting some groups but not others in the same generation. Berk gives an example of teenagers who witnessed the terrorist attack of September 11, 2001, and teenagers who lost parents. Researchers cannot include these adolescents in the study because they display different experiences and emotions. An example of cross-section design affected by cohort effects is “comparisons of 25-year-old cohorts, 50-year-old cohorts, and 75-year-old cohorts – groups born, reared and educated in different years.” In this case, the results do not demonstrate age-related changes.