System Migration to the Edge (Rasmussen)

Jens Rasmussen's model of how systems migrate toward the boundary of acceptable performance under three simultaneous pressures: (1) pressure toward least effort (workers optimize for efficiency), (2) pressure toward financial targets (management optimizes for cost), (3) pressure from the performance gradient (the boundary of unacceptable performance creates a wall that systems bounce against). The system settles into a dynamic equilibrium near the edge of the safety envelope, where normal operational variation can push it over the boundary into failure. The model explains why accidents cluster at the edge of acceptable performance rather than occurring randomly: the system is systematically pushed toward the edge by the same forces that make it efficient. NASA's operations before both Challenger and Columbia drifted to the edge of thermal protection margins. BP's operations before Deepwater Horizon drifted to the edge of well control margins. The drift is not malicious; it's rational response to persistent pressures.

When to use it

When investigating why a system that was previously safe became unsafe. When safety improvements don't stick. When production pressure consistently overrides safety culture. When designing resilient systems in competitive or resource-constrained environments.

How it can help

Provides a systems-level explanation for why safety interventions that don't address the underlying pressures will fail—the system will migrate back to the edge because the pressures remain. Interventions: (1) make the safety boundary visible and measurable, not just procedural, (2) create counter-pressures that push back from the edge (safety incentives, margin monitoring), (3) build 'hard stops' that prevent migration past critical thresholds, (4) monitor the trajectory, not just the current position—are you closer to the edge than last year? (5) address the three pressures directly rather than trying to override them with rules. The key: you can't prevent edge migration through procedures alone; you must change the pressure landscape.