“…How are we able to compute learning gains? Most researchers prefer to work with the achievement levels of the same students based on longitudinal data (Beckett, Tancredi, & Wilson, 2004;Dekkers, Bosker, & Driessen, 2000;Muthe´n & Khoo, 1998;Raudenbush, 1989;Sammons, 1995;Singer & Willett, 2003;Tymms, Merrell, & Henderson, 1997Willms, 1992). Longitudinal data, however, are not always available.…”