This study investigated the independent and interactive effects of task type and task complexity on English as a Foreign Language (EFL) learners' writing performance. Sixty-three intermediate EFL learners completed two writing tasks that varied in task type (narrative vs. decision-making) and task complexity (simple vs. complex). Task complexity was manipulated through reasoning demands using a within-subjects design, while task type functioned as a between-subjects factor. Task stimuli were presented as animated video clips, and participants engaged in pre-task planning by annotating the videos. Writing performance was assessed using seven measures of Complexity, Accuracy, and Fluency (CAF): Mean Length of T-unit (MLT), Dependent Clauses per T-unit (DC/T), Complex Nominals per T-unit (CN/T), Complex Nominals per Clause (CN/C), Error-Free Ratio (EFR), Correct Verb Forms (CVF), and Text Length (TXL). Repeated-measures Multivariate Analysis of Variance (MANOVA) revealed significant effects of task type, task complexity, and their interaction. Decision-making tasks produced significantly higher MLT, CN/T, CN/C, EFR, and CVF than narrative tasks, whereas narrative tasks yielded greater fluency. Increasing task complexity increased MLT, CN/T, and CN/C but reduced accuracy, with no significant effects on DC/T or fluency. Significant Task Type × Task Complexity interactions were observed for MLT, CN/T, and CN/C, indicating that the effects of task complexity on syntactic complexity varied across task types. The findings demonstrate that task design exerts selective rather than uniform effects on L2 writing performance and that different dimensions of CAF respond differently to task manipulations.
