Endogenous Masking: Why Degradation Reduces Its Own Observability

Assessment instruments are commonly evaluated by how accurately they predict or classify the systems they observe. This paper argues that a logically prior question has received comparatively little attention: whether an assessment instrument is structurally capable of representing the state transition that matters for the decision it is relied upon to support. The paper distinguishes two forms of masking. Exogenous masking arises from factors external to the degradation process and can in principle be compensated for. Endogenous masking arises when a system's own adaptive response preserves successful operation while progressively weakening the correspondence between observable behaviour and the underlying state. A system is therefore least observable not when it is healthiest, nor when it is closest to collapse, but when it is most successful at maintaining its required function despite ongoing degradation. Under these conditions an assessment instrument may become threshold-blind: unable to distinguish states that warrant different decisions by the party bearing the consequences. An ordered four-step procedure for evaluating instrument adequacy is developed and applied to independently developed instruments — from vital-sign monitoring during compensated shock to periodic inspection of bridges and buildings. The framework predicts both the failures and the successes of these instruments, suggesting a common informational structure rather than a domain-specific phenomenon. It proposes that the structural adequacy of assessment instruments should itself become an object of risk analysis.