Students often experience difficulties in understanding grouped data quartiles because they tend to apply procedures without comprehending the relationships among quartile positions, cumulative frequencies, and quartile formulas. This study aimed to analyze students’ strategy changes in determining grouped data quartiles through a microgenetic approach. This research employed a qualitative method using a task-based microgenetic study approach. The study was conducted at SMA Negeri 4 Jayapura during the second semester of the 2025/2026 academic year. A total of 41 students participated in a pretest to identify their initial abilities, from which three students representing high (S1), medium (S2), and low (S3) ability levels were selected through purposive sampling. Data were collected through a diagnostic pretest, graded microgenetic tasks, observations, and semi-structured interviews. Data were analyzed microgenetically by identifying problem-solving episodes, strategies, strategy changes, and critical moments. The findings revealed that students’ strategies developed through four stages: understanding quartile positions, using cumulative frequencies, determining quartile classes, and applying quartile formulas conceptually. Strategy changes occurred when students understood the role of cumulative frequencies in determining quartile positions and classes. It can be concluded that understanding cumulative frequencies served as a critical moment that promoted strategy changes from procedural use toward conceptual understanding in determining grouped data quartiles.
