"Mastery is a term that all educators use and believe they understand well. But when pressed to describe precisely what it means to 'master' a concept, skill, or subject, everyone has a different definition," assert Thomas R. Guskey and Eric M. Anderson. The father of the mastery learning approach, Benjamin S. Bloom, left the definition of mastery to individual teachers and schools, believing that decisions about what level of student performance represents true mastery involve value judgments and are best made at the local level. Although in many contexts mastery is defined as attainment of a specific percentage correct, in practice it's not that simple: a more important consideration is the cognitive complexity of the assessment questions students are required to answer or the difficulty of the tasks they are required to perform. Therefore, setting the level of mastery should always involve the thoughtful and informed professional judgment of educators, ideally working collaboratively.