Critics charged that Montessori's method was outdated, overly rigid, overly reliant on sense-training, and left too little scope for imagination, social interaction, and play.[50] In addition, Montessori's insistence on tight control over the elaboration of her method, the training of teachers, the production and use of materials, and the establishment of schools became a source of conflict and controversy. After she left in 1915, the Montessori movement in the United States fragmented, and Montessori education was a negligible factor in education in the United States until 1952