Controls for such studies are smudgy, as in all “soft” sciences, but I question/defy the “English Only” approach in the classroom, yet wondering if some consensus, if some meta-study is actually out there. As a language learner I have been in both immersion and mixed classrooms, was far more comfortable in the mixed AND feel I learned more. Plus I was raised bilingual Spanish/English, almost always in a mixed environment, zero confusion, speak both sin problema. As an instructor I let the students express themselves as they please, no pressure, avoid frustration as much as possible, use drills and games to illicit English, of course use English myself as much as possible, and often ask for translations/interpretations both directions to facilitate that skill and check for understanding (this last a particular concern, have run into some gnarly misconceptions beyond meaningless parroting stemming from immersion, gets into an unlearn one thing to relearn another which is at least prob three times the effort if not more). And, what’s more, I ask my students if my Japanese is acceptable in an effort to model language learning as well. I’d like to think I get at least the same results, and sometimes kid myself that I get better… but I have no data to back this up. A little help, please? Thanks (Arigatou)!