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http://books.google.co.kr/books?hl=ko&lr=&id=CIawG2sj3LMC&oi=fnd&pg=PP2&dq=ability+grouping+in+education&ots=13a7Pj7g1-&sig=g5ikJCEuRaDbAPyUjZKx8rIslas#v=onepage&q=ability%20grouping%20in%20education&f=false