Longitudinal design.
A research design where subjects are assessed at several different times in their lives. Usually, you use this design when you are interested in how subjects change over time. These studies are often expensive, difficult to conduct, and have lots of trouble without drop outs. They also will require more complex statistical analyses. But they provide a wealth of information that could not be obtained readily with other types of research designs. Here are two examples of longitudinal designs.
http://www.cmh.edu/stats/definitions/longitudinal.htm
John D
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