There are considerable and numerous obstacles to system­ atic classroom-based language learning research due to the complex nature of the classroom environment: Learners often have varying native languages and cultures , proficiency levels , learning styles , motivations, and attitudes; teachers have their own distinctive belief systems, teaching styles , preferred language, teaching materials, and professional and personal agendas . Furthermore, the work of these teachers and learners takes place in educational institutions that have their own sets of political , financial , educational , and social concerns. The goal of carrying out systematic and theoretically sound classroom studies can give rise to con­ flict between researcher and school personnel , leading to anything from minor personal friction to skewed results or aborted research plans .