Many differnt reasons.

Perhaps generalization, perhaps cross-validation, perhaps measure-equivalence.

Also, there is the prospect of empirically trialing new content (extending measures).

Hope this helps,

Matt

Hello Leanne,

Matt has given you some good suggestions above. However, I have read your question a number of times, and it seems to be wobbly in a number of respects, so it's a bit difficult trying to determine how best to help you. Perhaps come back and make things clearer.

That aside, people usually conduct longitudinal studies in order to see how things change over time. However, there could be a number of variables that influence change, so it is often handy to have appropriately chosen comparison groups to detect whether change might be occurring for the reasons the researchers are interested in, or perhaps for other reasons.

I'm pretty sure that if you just googled "Longitudinal research" you'd find some helpful information - i.e., information that suits your needs.