The central idea of dynamic assessment is to assess students’ potential to learn rather than measure what they have just done. Dynamic assessment focuses on the progress of the student. Testing acts as a diagnostic tool for a teacher to guide the student to successful learning, finding ways to overcome each person’s learning difficulties. Assessment and intervention combine in the process of dynamic assessment. At any point during learning, a student can demonstrate some skills and knowledge unaided. With the help of a teacher, or knowledgeable expert, the student can perform better. The psychologist Lev Vygotsky called the gap between what a student can do without help and what can be done with some personal assistance, the Zone of Proximal Development. This is the ‘zone of opportunity’, where a teacher can have the most success in improving the student’s learning. Dynamic assessment has been criticized on the grounds of its reliability.