This corpus study uses a quasi-experimental within-subjects design to compare student writing performance across collaborative and individual writing conditions using measures from the CAF (Complexity, Accuracy, Fluency) framework. Previous research suggests that collaboratively written texts are generally shorter and more accurate than texts written individually. However, it remains unclear which specific linguistic features are affected by collaborative writing (CW), possibly due to substantial variation in how written output has been measured across studies. The present study constructs a learner corpus to compare collaborative and individual writing using measures from the CAF framework. In addition, specific linguistic features, such as the accuracy of clausal subordination (e.g., conjunction use), are analyzed. The data are drawn from thirty-eight first-year Engineering students from two sections of an English academic writing course at a private Japanese university during the fall semester. Over the semester, students completed eleven in-class paragraphs collaboratively and eleven paragraphs individually for homework on different topics, resulting in a learner corpus of writing produced under both collaborative and individual conditions. The poster will also discuss methodological decisions involved in constructing the learner corpus, including issues related to digitization and annotation. Preliminary analyses of the data will be presented.