Dr. Robert Slater and his colleagues, through a partnership with the RAND Corporation, Portland Public Schools, and the American Councils for International Schools, conducted a four year study to compare dual language immersion students with their non-dli peers.
The Portland Study found that students randomly assigned to DLI programs outperformed their peers in reading in English by seven months in grade five and by nine months in grade eight.
The study concluded that DLI can provide schools and districts with the opportunity to support historically underserved students via an additive approach for ALL that leads to biliteracy.
Check out the article from the American Councils for International Schools.