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IDEF0 - Part 1 (understanding it)

Quality Tools > Tools of the Trade > 19: IDEF0 - Part 1 (understanding it)


There are a wide range of methods for diagramming processes, such as flowcharts and dataflow diagrams, but few which are both rigorously defined and are non-proprietary. SADT, or Structured Analysis and Design Technique was placed in the public domain in the late 1960s as IDEF0 by the US DOD as a standard for process definition. Since then it has been adopted by many organisations seeking a simple but rigorous way of describing their processes.


How does it work

IDEF0 is, in many ways, a very simple method. One example of this is that there is only one type of box in the methodology. Each box represents a single process, as with other approaches, but IDEF0 is different in the use and placements of arrows. As well as the conventional inputs and outputs, there are two other types of arrow which represent 'controls' and 'mechanisms'.


Controls are a form of input, but which are used to direct the activity in the process. There is occasionally some degree of uncertainty as to whether an item is an input or a control. A simple way of distinguishing these is that inputs get transformed or changed in some way in order to create the outputs, whilst controls seldom are changed. Standards, plans, templates and checklists are all forms of control.


Mechanisms are the resources and tools that are required to complete the process. This includes people with particular skills, machines and other tools.


The four arrow types, Inputs, Controls, Outputs and Mechanisms are collectively called ICOM and IDEF is an abbreviation of ICOM DEFinition. There is a zero on IDEF0 because there are a number of additional IDEF standards.


The different ICOM arrows are identified by the side of the activity box which they touch. Thus inputs are on the left, controls at the top, outputs on the right and mechanisms at the bottom. This can make diagrams a bit more difficult to draw but can make them more easily readable.



When there are many arrows in a diagram, they can make the diagram less readable, so arrows may be combined. The arrows are always named, so when they split out, it should be clear what they represent.


Each IDEF0 diagram consists of three to six activity boxes, with the ICOM arrows connecting the boxes.


Each activity box may be broken down to show the sub-process that it represents. When this happens, the ICOM arrows that enter and leave a box will usually appear in the decomposed process. This is not forced, and when arrows will not appear in the decomposed diagram, the arrowhead has parentheses placed around it. Similarly, when a new ICOM arrow is used in a diagram that was not shown in the parent diagram, parentheses are used at the start of the arrow. This is known as 'tunnelling'.


Numbering is used to help link diagrams together. Each diagram has a A-number. The Top-level diagram which contains a single activity box is called the A-0 (a minus zero) diagram. The next box is A0. Lower level diagrams are numbered according to the box from which they started, so the first box on the A0 diagram becomes A1, with the first box on that diagram becoming A11, and so on.


Each diagram also has a 'C-number', which takes account of successive versions of the same diagram. The C-number may be followed by the previous C-number in parentheses, to allow a historical linkage. ICOM lines are numbered from the top and left, so the leftmost control is C1, the next is C2 and so on. Each diagram




A general note about IDEF0: The small number of boxes on the diagram can result in a deep hierarchy, with a number of layers. This means that while individual process diagrams are easy to understand, the many diagrams that result from multiple breakdowns can be difficult to absorb as a whole. It is, though, a matter of swings and roundabouts, as in other methods which have many boxes on show at once, it can be difficult to understand single diagrams.


Next time: IDEF0 - Part 2 (doing it)


This article first appeared in Quality World, the journal of the Institute for Quality Assurance

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