Developmental research assesses variables over time, how people are different with different age/level of development. You can do this through a Longitudinal Design where same people are studied over a long period of time (e.g. follow 20 five-year-olds for ten years to see how their cognitive functioning has changed). However, people tend to drop-out or learn about the purpose of the study. In a Cross-Sectional Design, you take different individuals from different age groups and study them (e.g. take 20 five-year-old, 20 ten-year-olds, etc and see the differences). The problem with this design is that different cohorts have different experiences and growing up in experienced differently with each cohort. Lastly, in a Cross-Sequential Design, you take different individuals from different age groups and study them for a short period of time. It combines elements from longitudinal and cross-sectional designs. In a Time-Series Design, individuals from one or more groups are studied at different times for the same variable.