PSL/PSA is perhaps the world's oldest and most comprehensive set of tools for stating system requirements. It started in 1968 and has had a very impressive set of sponsors at various times. Read about it in HISTORY.

The history of CASE (Computer Aided/Assisted Software/System Engineering) shows that since the mid-1980s the evolution of the market for tools moved substantially in the direction of PC-based graphical techniques using methods such as data flow diagrams and data models. There is a view that these PC-based graphical techniques are generally only useful for relatively trivial systems, and when used for large (size or complexity) the techniques will be dysfunctional by rendering the proper checking of completeness and consistency either extremely difficult or impossible. Linguistic techniques such as PSL/PSA have no known limit of scalability (there is anecdote of one system modelled with PSL/PSA handling around 35,000 objects and 65,000 relations). Unfortunately there has been very little research in this area, so even today the choice of method is more likely to be based on familiarity, prejudice or ideology rather than supportable evaluation of alternatives.

PSL/PSA was the best-known output from the ISDOS (Information System Design and Optimization System) Project at the University of Michigan (UoM) under Professor Daniel Teichroew in the Department of Industrial and Operations Engineering. That project was sponsored by many substantial organizations.

Most ISDOS products were developed at UoM between 1968** and about 1983/4. Then a separate commercial operation (ISDOS Inc) was set up. Then commercial operations were separated from UoM and went through a series of ownerships as described briefly in COMMERCIAL OPERATIONS.

There are three sections below this page:

• ISDOS - covering UoM activities
• Commercial Operations covering exploitation of ISDOS IPR outside of UoM
• Future - what next for PSL/PSA and related tools

**The ISDOS Project commenced initially during 1967 in the School of Management at Case Western Reserve University, Cleveland, Ohio. It moved to the University of Michigan in 1968. The terms 'Problem Statement Language' and 'Problem Statement Analyzer' are used right from the beginning in ISDOS Working Paper No. 1 (1967).

Additional tools have been developed alongside PSL/PSA and independently - see TOOLS.