Zone of Proximal Development-ZPD is a concept originated from Lev Vygotsky, a social psychologist. ZPD emphasis learning as a social phenomenon. It is a range within which the children can learn. It is the most productive zone for learning. At this range students are learning task that they can complete independently; at the other end, they are learning task that they can not complete, even with assistance. In between these two extreme zones is ZPD, the range of tasks in which children can achieve if they are assisted by more knowledgeable or more competent other.
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http://www.docstoc.com/docs/27395530/Quotes-from-Lev-Vygotsky-on-the-Zone-of-Proximal