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PSL/PSA - Past

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.

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. After that, commercial operations were separated from UoM and went through a series of ownerships as described briefly in COMMERCIAL OPERATIONS.

There are additional sections to be added to the website:

  • ISDOS - covering UoM activities
  • Commercial Operations covering exploitation
  • Future - what next for PSL/PSA
  • 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

The language evolved from 1968-1991. Key version ranges were:

Date Range Version(s)
1968 1.0
1970 2.0
1975 3.0
1977 4.2
- 1981 5.0 - 5.2
1987 - 1991 6r0 - 6r3
1991 - 2011 use on various large projects
2012 on - 7.0 -

The major phases of the ownership of PSL/PSA are shown in the following table:

Date Range Ownership of PSL/PSA
1968 - circa 1983 University of Michigan ISDOS Project
circa 1983 - circa 1987 ISDOS Inc.
circa 1987 - circa 1990 Meta Systems Ltd.
circa 1990 - 1996 LBMS Ltd.
1996 onwards Geoffrey Darnton

The last release generally available commercially was PSL/PSA Version 6.0 Release 3 around September 1990.

A simplified version of Problem Statement Language (PSL-lite) will be made available, and the language placed in the public domain (along with the original PSL). Commercial exploitation can be done by anyone who wishes to implement the language. Copyright will be retained to ensure a mechanism to certify compliance and prevent abuse. A new full version of PSL will be available commercially.

PSL/PSA, Related Tools and associated training can be provided now on reasonable commercial terms to anyone who has a need or interest to use them.


Would you like a model or comprehensive description of your business or organization? Many organizations would. The best way to do that is have your own 'language' to use for describing your business, or even just part of it. This can be critical for tracking many different components of the business, looking for gaps and inconsistencies.

Read more: metaPSL®

PSL/PSA Syntax, Components and Addons

This section presents the syntax of PSL/PSA v6.0 as it was when it was last available as a commercial product. It also presents information about additional tools and addons that were developed to supplement the main functionality of PSL/PSA.


The Objects of PSL v6.0 are:

  • Attribute
  • Condition
  • Element
  • Entity
  • Event
  • Group
  • Input
  • Interface
  • Memo
  • Output
  • Process
  • Processor
  • Relation
  • Requirement
  • Resource
  • SET
  • Subtype
  • System-Parameter
  • Unit

Depending on how you group relations to classify them, PSL v6.0 has approximately 100 relations.


PSL/PSA-light will be available very soon. It is being developed using a subset of the original PSL/PSA. So much of the original PSL/PSA was used rarely, and included for very specific sponsor needs. It has been designed to provide the objects and relations used in most projects. Sign up and express your interest so you can hear first.

Read more: PSL/PSA-lite


As soon as the first version of PSL/PSA-lite has been released, work will continue to make available once again, the full capability of PSL/PSA® as most people will have known it. There may be some variation and work is underway to investigate the reasoning behind the inclusion of some of the original relations.

Read more: PSL/PSA®

© Geoffrey Darnton 1992-2018