A growing number of safety-critical industries agree that building confidence in complex systems can be achieved through evidence and structured argumentation framed in assurance cases. Nevertheless, assurance cases can easily become too rigorous and difficult to develop and maintain when applied to complex systems. Therefore, we propose to use contract-based development (CBD), a method to manage complexity originally developed in computer science, to simplify assurance cases by modularizing them. This paper will not only summarize relevant previous work such as constructing consistent modular assurance cases using CBD, but more importantly also propose a novel approach to integrate CBD with the argumentation in assurance case modules. This approach will allow interdisciplinary subject-matter and domain experts to build assurance cases together without even knowing about CBD. This helps subject matter experts outside of computer science to reap benefits from CBD and helps with interdisciplinary co-development of assurance cases that cover all the required fields. This paper motivates four rules of thumb aimed to help practitioners developing high-quality modular assurance cases. It also explains how modularization of assurance is an enabler for multi-concern assurance that accounts for the inter-dependency of different concerns such as safety, security and performance.

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