Chapter 8 presents the rationale for and the research design of the case study which constituted the qualitative part of the sequential mixed-method (MM) study. According to CDST, language development is an individually-owned process in which learners follow unique developmental paths. In line with the ergodicity principle, research results should not be generalised from the group to the individual, and vice versa, unless the group is an ergodic ensemble. Thus, the aim of the case study was to examine both intra-individual variability and inter-individual variation in the emergence of syntactic complexity, lexical complexity, accuracy, and fluency (CALF) in L2 English writing at secondary school in the case of selected good, average, and poor language learners. The case study was conducted in 2014–2017 at a secondary school in Czestochowa, Poland. It was based on a subcorpus which was subsampled from the Written English Developmental Corpus of Polish Learners (WEDCPL) and included 189 texts (38,803 words) written by nine learners during 21 repeated measurements. The participants, aged 16–19, were at B1 and B2 levels in grades 1–3. Their selection was based on a pre-test comprising placement, writing, and speaking tests.