Essential Systems Analysis

Essential Systems Analysis was a new methodology for software specification published in 1984 by Stephen M. McMenamin and John F. Palmer for performing Structured Systems Analysis based on the concept of Event Partitioning.
The Essence of a system is "its required behavior independent of the technology used to implement the system". It is a model of what the system must do saying ideally nothing about how it will do it.
The methodology proposed that finding the true requirements for an information system entails the development of an Essential Model for the system, based on the concepts of a perfect internal technology, composed of:
It was later adapted by Edward Yourdon to develop Modern Structured Analysis.
The main result was a new and more systematic way to develop the Data Flow Diagrams, which are the most characteristic tool of Structured analysis|Structured Analysis.
Essential Analysis, as adopted in Yourdon´s Modern Structured Analysis, was the main software development methodoly until Object Oriented Analysis became mainstream.