Experimental Designs in Developmental Psychology

Longitudinal Designs
Examine developmental changes in one cohort followed over time
Advantages:
Process of development can be followed with individuals
Disadvantages:
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
Advantages:
Fast and cheap
No subject attrition
Disadvantages:
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.