Lehman's Laws of Software Evolution - The Laws

The Laws

All told eight laws were formulated:

1. (1974) Continuing Change — E-type systems must be continually adapted or they become progressively less satisfactory.
2. (1974) Increasing Complexity — As an E-type system evolves its complexity increases unless work is done to maintain or reduce it.
3. (1974) Self Regulation — E-type system evolution process is self-regulating with distribution of product and process measures close to normal.
4. (1978) Conservation of Organisational Stability (invariant work rate) - The average effective global activity rate in an evolving E-type system is invariant over product lifetime.
5. (1978) Conservation of Familiarity — As an E-type system evolves all associated with it, developers, sales personnel, users, for example, must maintain mastery of its content and behaviour to achieve satisfactory evolution. Excessive growth diminishes that mastery. Hence the average incremental growth remains invariant as the system evolves.
6. (1991) Continuing Growth — The functional content of E-type systems must be continually increased to maintain user satisfaction over their lifetime.
7. (1996) Declining Quality — The quality of E-type systems will appear to be declining unless they are rigorously maintained and adapted to operational environment changes.
8. (1996) Feedback System (first stated 1974, formalised as law 1996) — E-type evolution processes constitute multi-level, multi-loop, multi-agent feedback systems and must be treated as such to achieve significant improvement over any reasonable base.