Robert de la SernaThis study investigates the effectiveness and efficiency of mathematics instruction in two languages (English vs. Cebuano/English Code-Switching) on the performances of Filipino algebra students in 11 classes of a Philippine private high school. Conducted in 2005-2006, a quasi-experiment addressed the question: Between the two languages of instruction, which promotes better algebra learning among Filipino bilingual students’ The analyses are limited to evaluating the performances of high-ability students (comprising 3 classes) by using Multi-level Nested Modelling to compare the impact of languages-in-use on student achievement, and Data Envelopment Analysis (DEA) “a novel economic modelling approach” to measure the relative efficiencies of student learning outcomes.