Theory 02/Repair capacity/Biology and engineering
The Maintenance Ratio
The failure rate of a maintained system is set not by its condition but by the ratio of its degradation rate to its repair capacity, so two systems with identical maintenance budgets can fail at very different rates if their degradation rates differ.
01What is already known
Reliability engineering has long used condition-based and interval-based maintenance, and both assume that maintenance effort maps onto reliability. Medicine makes a similar assumption about physiological reserve, and ecology about resilience. The observed pattern is messier: some well-maintained assets fail early, and some neglected ones persist for decades.
02The mechanism
Degradation removes capability at a rate set by use and environment. Repair restores it at a rate set by capacity — labour, perfusion, nutrient supply. The system's condition is the integral of the difference. When the ratio of the two rates is near one, condition hovers and failure is driven by variance rather than by trend; when it is far from one, the system either fails quickly or effectively does not age. The critical property is that the ratio, not the level, determines which regime you are in.
03Predictions
- Failure time distributions should cluster by degradation-to-repair ratio rather than by age or by maintenance spend.
- Raising repair capacity in a system already above the critical ratio should have little effect on failure rate.
- Systems engineered to reduce degradation should outperform systems given more maintenance, at equal cost.
04Refutation
If failure rate tracks condition independently of the ratio — that is, if a system at condition X always has the same hazard regardless of how fast it is degrading — the ratio adds nothing.
Longitudinal condition data with known intervention history, stratified by estimated degradation rate. The ratio hypothesis predicts that hazard at a given condition differs systematically between strata.