Forcing Functions

A strong constraint that makes it impossible, or at least hard, to proceed until a problem is corrected. Three types: interlocks force a sequence, lock-ins keep an operation active, and lock-outs prevent entering a dangerous state.

Why it Matters

For high-cost errors, prevention beats correction. A forcing function stops the error from happening rather than relying on the user's attention or memory.

Signals

• Dangerous or irreversible actions reachable with a single slip.
• Errors that recur despite warnings; safety steps people routinely skip.

Benefits

Serious errors become structurally impossible, with far less reliance on vigilance.

Risks

Over-applied, forcing functions become annoying and get bypassed or disabled, which defeats their purpose.

Tensions

Safety versus convenience: every barrier that prevents a mistake also slows the expert who knew exactly what they were doing.

Examples

A microwave that will not run with the door open (interlock and lock-out); an "are you sure?" confirmation on permanent deletion; needing the clutch down to start a car.