Chapter 6 presents the results of the panel study, the aim of which 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 results indicated that the learners progressed in language development, but that they developed lexical complexity and fluency at the cost of syntactic complexity and accuracy, nominalisation at the cost of subordination and coordination, and lexical variation at the cost of density and sophistication. Standard and moving correlations showed some competition between complexity, accuracy, and fluency, but support within syntactic and lexical complexity. The analysis of intra-individual variability revealed hardly any significant peaks and phase transitions. However, Monte Carlo analyses revealed some statistically significant differences in intra-individual variability between complexity, accuracy, and fluency. The results of the average semester growth rates (ASGRs) showed that fluency developed more rapidly than complexity, which in turn developed more rapidly than accuracy. Furthermore, nominalisation and variation developed more quickly than the remaining syntactic and lexical variables, respectively. Finally, it was established that intra-individual variability had a positive influence on the average semester growth rates of most of the CALF variables.