Chapter 5 presents the rationale for and the research design of the panel study which constituted the quantitative part of the sequential mixed-method (MM) study. MM designs combining group-based and individual-based data are recommended for CDST research. Panel design involves measuring the same variables in the same learners over time. So far, the role of intra-individual variability in language development has been examined mainly in longitudinal case studies, involving few participants, at the university level. The aim of the study was to examine the influence of intra-individual variability on the average semester growth rates (ASGRs) of syntactic complexity, lexical complexity, accuracy, and fluency (CALF) in L2 English writing at secondary school. The study was conducted in 2014–2017 at a secondary school in Czestochowa, Poland. It was based on the Written English Developmental Corpus of Polish Learners (WEDCPL), which consists of 1923 essays written by 100 learners during 21 repeated measurements (CECL, 14 October 2024). The participants, aged 16–19, represented B1 and B2 levels in grades 1–3. The study examined 11 variables: (1) syntactic complexity, including subordination, coordination, and nominalisation; (2) lexical complexity, including lexical density, sophistication, and variation; (3) accuracy and fluency; and (4) general language development.