Hubungan antara latar belakang keluarga dengan prestasi belajar anak berbakat di kelas unggul SLTP Negeri di Sumatra Barat (Studi anak berbakat di SLTP N Padang, Padang Panjang dan Bukittinggi) = Relationship between family background and leaming achievement of gifted students in a 'smart class' of a state junior High School (SLTP N), West Sumatera. (The study of gifted students at state junior High School's in Padang, Padang Panjang and Bukittinggi)