To meet the growth in learner diversity in today's classroom, a new paradigm for improving the precision and accuracy of "personalization" is critical to address the needs of students who are held back by traditional pathways designed for the mythical "average" learner. In response to this challenge, Digital Promise Global has embarked on a multi-year initiative, Learner Positioning Systems™ (LPS). The culmination of two years of systematic literature review and framework development, the LPS website is a free, open-source digital resource presenting the research behind the Learner Factors ("Factors") that most impact learner variability. The LPS app translates the Factor research into actionable educational product design and curriculum strategies for developers and educators to support research-based personalization for the full diversity of learners. This paper provides an overview of the research behind LPS's first area of focus, K-3 Reading. Beginning with the common standards used to evaluate reading success, the paper then details each area of LPS's K-3 Reading research and identifies the specific Factors with links to full annotated bibliographies for each. Finally, this paper explores interactions among Factors to highlight the complexity inherent in each person's individual learning path.