Observing that most software is subject to change in the course of its existence, the authors set out to determine laws that these changes will typically obey, or must obey in order for the software to survive. In his 1980 article, Lehman qualified the application of such laws by distinguishing between three categories of software:
An S-program is written according to an exact specification of what that program can do
A P-program is written to implement certain procedures that completely determine what the program can do
An E-program is written to perform some real-world activity; how it should behave is strongly linked to the environment in which it runs, and such a program needs to adapt to varying requirements and circumstances in that environment
The laws are said to apply only to the last category of systems.
"Continuing Change" — an E-type system must be continually adapted or it becomes progressively less satisfactory.
"Increasing Complexity" — as an E-type system evolves, its complexity increases unless work is done to maintain or reduce it.
"Self Regulation" — E-type system evolution processes are self-regulating with the distribution of product and process measures close to normal.
"Conservation of Organisational Stability " — the average effective global activity rate in an evolving E-type system is invariant over the product's lifetime.
"Conservation of Familiarity" — as an E-type system evolves, all associated with it, developers, sales personnel and 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.
"Continuing Growth" — the functional content of an E-type system must be continually increased to maintain user satisfaction over its lifetime.
"Declining Quality" — the quality of an E-type system will appear to be declining unless it is rigorously maintained and adapted to operational environment changes.
"Feedback System" — 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.
