The Interference Diagram (ID) is a relatively simple tool to help surface, value and sort out interferences, Undesirable Effects or obstacles usually described as “problems”. The ID found its place within the Theory of Constraints Logical Thinking toolbox, but “Unlike the other thinking tools the ID is not based on logic, but rather on intuition(1)”.
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How does it work?
Gathering a group of people au fait with issues and the processes involved, the Goal to be achieved is written in the center of a board or paper sheet.
Every participant is asked to list the interferences he/she encountered, which is anything hindering the system to achieve the Goal.
Interference Diagram as I understand it is not a brainstorming tool, rather a mind mapping tool. The difference is about looking things that happened rather than think that could happen. The purpose of the ID is to understand and focus on unsolved problems that keep the system from achieving its Goal. This is important for the next step which tries to evaluate the impact of each interference based on the experience and memory of the participants.
These interferences are listed and written around the Goal.
Each interference, unless unvalidated by participants, is valued for its impact on the Goal, e.g. how much does it impair the system achieving its Goal.
Time is a convenient unit as it is common to everyone and every activity. Time can be later converted in other units like number of widgets potentially lost (lost time / unit cycle time, for example).
Once the interferences valued, the list can be ordered from highest impact to least, showing a Pareto diagram and giving hints about interferences to cope with in priority, which commonsense says are the most impacting ones(2).
When to use the Interference Diagram?
I found the ID convenient to overcome the lack of available data, for example when a machine has a low OEE but no data recording can break down the causes of poor OEE.
The ID is therefore a convenient tool in case of problems but few, messy data.
(1) For more information about ID (origins, usage, etc), see: http://focusandleverage.blogspot.fr/search?q=ID+diagram
(2) I have a slight reservation about focusing solely on a Pareto Head, find out why >here<
(3) Epiphanized: Integrating Theory of Constraints, Lean and Six Sigma (TLS), Bob Sproull & Bruce Nelson, North River Press, 2012