This reference document is a companion to the Language Complexity Study conducted by RTI International under the All Children Reading–Philippines project in 2020. The study was a secondary analysis of Early Grade Reading Assessment (EGRA) data at multiple time points, looking specifically at the effect of language complexity on reading acquisition of second and third languages (L2 and L3, respectively). The study used the 2013 and 2019 National EGRA data sets to analyze performance according to categories of language complexity based on syllabic complexity, orthographic depth, and other related items. The study was guided by the methodology described by Brunette et al. (2019), who studied the effects of language complexity on reading outcomes in Uganda. The languages selected for analysis were those among the officially supported mother tongue (MT) languages of instruction in the Philippines. Analysts assessed whether the level of complexity was in any way predictive of the average increase or decrease in L2 and L3 scores across schools as measured in 2019 (provided there were schools that reported using MT in 2019, which was not the case for three of the languages: Chavacano, Ivatan, and Sambal.