Assurance Argument

An Assurance Argument is a reasoned argument that seeks to justify, or to rebut, an assurance claim within an assurance case.

AKA: Assurance Case Argument.
Context:
- It can (typically) link Assurance Evidence Items and explicit assumptions to the claim it supports.
- It can infer a higher-level claim from one or more subclaims, with no direct evidence cited at that step.
- It can be documented with an explanation of its reasoning where the inference from premises to conclusion is not obvious.
- It can be assessed for validity separately from whether the claim it argues for is true.
- It can be used against a claim, as an argument that rebuts it rather than one that supports it.
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- It can range from being a Supporting Assurance Argument to being a Rebutting Assurance Argument, depending on whether it argues for or against its claim.
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Example(s):
- an argument that a system enforces its security policy because every access path is mediated and every mediation point has been tested.
- an argument that infers a top-level security claim from subclaims about a system's requirements, design, implementation and maintenance.
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Counter-Example(s):
- a Legal Domain-Specific Reasoned Argument about a contract dispute, which is not made within an assurance case.
- an Assurance Claim, which is the proposition argued for rather than the argument.
- an Assurance Evidence Item, which an argument cites but which is not itself an argument.
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See: Reasoned Argument, Assurance Claim, Assurance Evidence Item.

References

Ross, R., Winstead, M. & McEvilley, M. (2022). Engineering Trustworthy Secure Systems, NIST Special Publication 800-160 Volume 1, Revision 1. Retrieved 2026-10-15. Supports the definition and the evidence link: an assurance case contains "Arguments that logically link the evidence and any assumptions"; an effective security assurance case contains "valid arguments that relate the various evidence to the supported security claims"; "Top-level claims decompose in a structured manner into subclaims".
Object Management Group. (2022). Structured Assurance Case Metamodel (SACM), Version 2.2. Retrieved 2026-10-15. Supports claim-to-claim inference and documented reasoning: "The core of any argument is a series of claims (premises) that are asserted to provide sufficient reasoning to support a (higher- level) claim (a conclusion)"; "ArgumentReasoning can be used to provide additional description or explanation of the asserted relationship".