Experimental Designs in Developmental Psychology

Longitudinal Designs
Examine developmental changes in one cohort followed over time
Process of development can be followed with individuals
Large investment of time and money is required
(especially if the age span of interest is large)
Subject attrition can be a problem
Carryover effects (e.g., learning) can be a problem
Differences among cohorts are not addressed

Cross-sectional Designs
Examine two (or more) ages (or cohorts) at one time
Fast and cheap
No subject attrition
Confounds age and cohort effects
Unable to examine the process of development within individuals

Cohort-Sequential (Cross-sequential, time-sequential) Designs
A combination of the above; two (or more) cohorts are each studeis at two (or more) ages. (Sometimes with additional groups tested once to "fill in" the design.)

Advantages & Disadvantages
This is a compromise between the longitudinal and cross-sectional designs. It will have some of the advantages and disadvantages of each, depending upon the length of the within cohort component and the number of different cohorts studied.