Intent Avoid radical changes that may alienate users.
How do you accomplish a major overhaul of a legacy system without disrupting the way users are used to getting their job done?
This problem is difficult because:
- The legacy system requires significant changes.
- The users are not happy with the legacy system, but they understand it well.
Yet, solving this problem is feasible because:
- You can migrate incrementally to a new solution.
Introduce only a constant, relatively low number of changes between each new release.
- Users do not have to change their work habits too much between releases.
- Sometimes radical change is necessary. It can be hard to migrate from a command-line interface to a GUI while conserving familiarity.
Too much change between releases increases the risk of hidden defects, and decreases the chance of user acceptance.
Lehman and Belady’s “Law of Conservation of Familiarity” suggests that the incremental change between releases of a system stays roughly constant over time [LB85]. This is a relatively natural phenomenon because to do anything else introduces unnecessary risks.
To Conserve Familiarity you must Migrate Systems Incrementally. Involve the Users to understand what changes will be acceptable. Prototype the Target Solution to evaluate the potential impact of changes.