A 2017 review on evaluations of Montessori education studies states that wide evidence exist that certain elements of the Montessori method (e.g. teaching early literacy through a phonic approach embedded in a rich language context, and providing a sensorial foundation for mathematics education) are effective. At the same time it was concluded that while some evidence exist that children may benefit cognitively and socially from Montessori education which sticks to original principles, it is less clear whether modern adapted forms of Montessori education are as effective. Lillard (2017) also reviews research on the outcomes of Montessori education.
A 2006 study published in Science magazine found that "when strictly implemented, Montessori education fosters social and academic skills that are equal or superior to those fostered by a pool of other types of schools." The study had a relatively small sample size and was severely criticized. Another study in the Milwaukee Public Schools found that children who had attended Montessori from ages 3-11 outperformed their high school classmates several years later on mathematics and science; another found that Montessori had some of the largest positive effects on achievement of all programs evaluated.
Some studies have not found positive outcomes for children in Montessori classrooms, but this might be due to the implementation of Montessori. For example, a 2005 study in a Buffalo public Montessori magnet school "...failed to support the hypothesis that enrollment in a Montessori school was associated with higher academic achievement." Explicitly comparing outcomes of Montessori classrooms in which children spent a lot of time with Montessori materials, less time with the Montessori materials, or no time at all with the materials (because they were in conventional classrooms), Lillard (2012) found the best outcomes for children in classic Montessori.