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IDEF0: How to understand it

The Quality ToolbookIDEF0 > How to understand it

When to use it | How to understand it | Example | How to do it | Practical variations


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How to understand it

There are a number of methods available for describing processes, many of which are fairly informal, such as Block Diagrams or Flowcharts. These suffice in many circumstances but are inadequate in others, typically where the process is complex or a more rigorous approach is required. A problem which can arise when being more rigorous is that the overall understanding can be lost in the detail. The challenge is thus to find a method that is simultaneously precise and lucid at all levels.

IDEF0 (pronounced 'eye-deff-zero') provides a formal method of describing processes or systems, using several techniques to avoid the complex diagrams that could result from an attempt at a complete description using other methods.

The basic diagram element is very simple, using just one box shape to define each activity or process, as illustrated. The four arrows around the box, whose initials give rise to the name ICOM codes, are:

  • Inputs, which are the 'raw material' that gets transformed during the activity, e.g. a wire coil, outline plans.

  • Controls, which influence or direct how the process works, e.g. safety standards, customer requirements, project plans.

  • Mechanisms, which cause the process to operate, e.g. people, tools, machines.

  • Outputs, which are the result of the activity and are transmitted to other processes, e.g. cut lengths of wire, final plans.


Fig. 1. ICOM arrows


(Note that ICOM is sometimes revised to ICOR, where the M for Mechanisms is revised to the easier-to-understand R for Resources.)

The process description starts with single box, showing the ICOM codes for the overall process. This diagram is called the 'A-0' diagram (pronounced 'A minus 0'), as illustrated in Fig. 2.



Fig. 2. A-0 Diagram


The box (or node) in the A-0 diagram is then decomposed (or 'exploded' or 'zoomed') into a diagram with between three and six boxes, as illustrated. This is called the 'A0' diagram.

This hierarchical decomposition is repeated for each box in this diagram, then for each box in the resultant diagrams and so on, until the process is fully described.

In any diagram, the boxes are generally laid out from the top left to the bottom right, in order of dominance, where a higher dominance box has a greater influence over a lower dominance box. Boxes are numbered in order of dominance, with the number being used to reference the box on other diagrams. Thus the first box in the A0 diagram is decomposed in diagram A1, and the third box in that diagram is decomposed as A13.

Arrows carry multiple items, and are merged or split to simplify the diagram. Names next to arrows should describe what they carry; where there are no names, the arrow contents may be deduced. Arrows which do not connect to a box at one end are those that come from or go to the parent box, from which this diagram is decomposed. These arrows are numbered to indicate which ICOM arrow they represent. Thus, C1 represents the leftmost control and I2 is the second input down on the parent box. Parentheses (a 'tunnel') at one end of an arrow indicates that this arrow does not appear on the parent or child diagram.



Fig. 3. A0 Diagram


There are only five types of connection that arrows can make between boxes, as indicated in the table below, which show how activities feed, enable or constrain one another. The mix of connection types in one set of diagram will indicate the overall system type. For example, a process with little feedback may indicate a lack of control.





Input connection

Output to input of lower dominance box,
e.g. assembly line

Control connection

Output to control of lower dominance box,
e.g. plans, specifications

Output mechanism

Output to mechanism of lower dominance box, e.g. setup, allocation

Control feedback

Output to control of higher dominance box,
e.g. reviews

Input feedback

Output to input of higher dominance box,
e.g. rework


One of the strengths of IDEF0 is that not only are the diagrams defined, but the process of how they are produced is also defined. A key part of this is that it is drawn by an IDEF0 expert (the 'author'), using the viewpoint of and with the full cooperation of a key individual within the system being diagrammed. This, along with formal authoring and reviewing cycles, helps to ensure consistent and correct detail in the final diagrams.

The completed model includes additional text descriptions of items in diagrams to enable the diagrams to be kept relatively light while maintaining a depth of information that can be used as required.



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