* The numbers in the table are rounded.
* ACTFL OPIc: Oral Proficiency Interview by Computer (OPIc) created by Language Testing International (LTI). LTI is the exclusive licensee of the American Council on the Teaching of Foreign Languages (ACTFL) link .
* italki: this is a special service that connects users to teachers and tutors; it is not a stand alone language app.
* 95% CI: 95% confidence interval with Agresti-Coull correction.
* At Least One Level Up: increased OPIc oral proficiency by at least one level (level definition link).
* Two Levels or more Up: increased OPIc oral proficiency by at least two levels.
* NA: Not directly comparable because the study time was self reported.