This paper investigates measures of change to help demonstrate the necessity of variability as a developmental mechanism for advancing different features of L2 learning (related here primarily to writing, but also to reading) with a particular focus on learners at different stages of development. To do so, the work draws on three studies to build a case for using variability as a meaningful marker of change. Lowie, Wander M. & Marjolijn Verspoor. 2019. Individual differences and the ergodicity problem. Language Learning 69. 184–206 found in a group of 22 Dutch learners of English that the Coefficient of Variation (CoV), rather than individual factors such as motivation and aptitude, showed a significant correlation with writing proficiency gains. A replication study by Huang, Ting, Rasmus Steinkrauss & Marjolijn Verspoor. 2020b. Variability as predictors for L2 writing proficiency. Journal of Second Language Writing , with 22 Chinese learners of English revealed that the CoV rather than motivation, aptitude or working memory was a significant predictor in writing proficiency gains. A study by Gui, Min, Xiaokan Chen & Marjolijn Verspoor. Submitted. The dynamics of reading development in English for Academic Purposes, on reading for academic purposes with 27 Chinese Chemistry majors showed that the Standard Deviation of differences (SDd) rather than proficiency in English or knowledge of Chemistry correlated with reading gains. Two further studies present tentative evidence that these changes take place especially at transitional phases while learning a new skill.