Please review the link provided, specifically in the section on Single-Group design. Maturation is a major concern with younger participants if the study is done over a period of time. Their views and knowledge change. To combat that, we try to have shorter studies or those that do not require the same feeling level. The same can be said with information in general. The longer the study the more history can evolve or change. What are your views?
Threats to validity of Research Design