The Trial Friction Index: A Working Specification for Burden-Triggered Protocol Branching

The Release Valve Model proposes prospectively authorised lower-burden data-collection pathways that can be activated when participation becomes difficult to sustain. That architecture requires a trigger that is sufficiently structured to be auditable, while remaining sensitive to forms of friction that may be invisible in ordinary compliance data. This paper specifies a working version of that trigger, the Trial Friction Index (TFI), drawing on domains represented in the validated Perceived Research Burden Assessment (PeRBA) while narrowing the construct to point-of-concern attrition friction rather than comprehensive research burden. The proposed instrument contains an unscored safety gate, an unscored disclosure flag, three scored experienced-friction items, and a separately recorded trajectory item assessing projected sustainability. No activation threshold is published; the paper defines a validation programme for deriving one and presents the TFI as a working specification, not a validated instrument.