This paper emphasizes the importance of task grouping in multi-task learning and proposes a novel metric to identify optimal task groups by measuring the relatedness between tasks. In particular, we present a method to measure the difficulty of tasks based on pointwise V-usable information (PVI) and group tasks with statistically similar PVI estimates. Through experiments on 15 NLP datasets from general, biomedical, and clinical domains, we verify the effectiveness of the proposed method compared to existing methods including Llama 2 and GPT-4. The experimental results show that tasks grouped based on PVI achieve competitive performance with fewer parameters and show consistent performance across domains.