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The Interference Diagram

Submitted By: Bob Sproull

For those of us engaged in performance improvement initiatives there seems to be a constant bombardment of “things” that seem get in the way of what we’re trying to accomplish.  Things that interfere with our attempts to achieve a goal or objective in our quest to make things better.  Some of these things come out of nowhere in the name of uncertainty to stifle our efforts and still others are there just waiting to be found and acted upon.  Wouldn’t it be great if there was a way, or at least a tool, to help visualize these “things” so that we could do something about them?  Life would be so much easier wouldn’t it?  Good news….such a tool does exist for identifying the myriad of business, production, healthcare and manufacturing issues that face us every day.  It’s called an Interference Diagram (ID).

Most of you have probably never have seen or even heard of an ID before.  Its origins date back to the mid-1990’s with Bob Fox and the TOC Center in New Haven, Connecticut.  As a thinking tool it has not been well publicized and as such, is not well known.  But just because it hasn’t been well publicized, don’t underestimate its importance from a problem solving perspective. The endearing qualities of the ID is that it’s both simple to learn and construct and is quite robust in its application.  The ID is a thinking tool that offers the capacity to define and visualize those interferences or obstacles that block or hinder your ability to achieve a specific goal or outcome.  It’s always far easier to define what we want, but much more difficult to define why we can’t have it and the ID helps us do that.

The ID can be used at many different organizational levels to understand why things at all levels don’t happen or work the way we want them to.  The ID can be used as a stand-alone tool or it can be used in conjunction with other tools, so its uses are multiple.  As a stand-alone tool it provides a discrete analysis to better define and understand the obstacles that prevent accomplishment of our goal. But in a broader application it can be used to supplement the other, more common systems thinking tools developed by Dr. Goldratt as well as his Five Focusing Steps.  No matter which way it is used, the end results can be very dynamic.

The Interference Diagram is quite simple to construct as depicted in Figure 1.  The first step in its construction is to decide what it is you want more of or what your goal or objective is.  When you’ve decided what that is, write it inside the circle in the center of a white board or piece of paper.  Make sure that whatever you write here is a succinct, precise statement so that it’s easy to work with.


After you’ve considered and recorded what it is you want more of, think to yourself “What prevents me from getting more of what I want?” The answer to this simple question becomes the interferences that you record in the boxes surrounding your goal.  Continue to list your interferences until you are satisfied that your list is complete enough to move on.  There are really no rules that govern a specific number of interferences you can list, just be sure to list the major interferences as to why you can’t achieve what it is that you want more of.  Make sure you include things that you might think you can’t do anything about like lunches and breaks.

If your ID analysis is such where time is an important factor, then it is important to quantify your interferences as a function of time.  In other words, how much time does this particular interference take away from what you want?  If time is a factor, and most of the time it will be, then it’s important to keep the time element constant.  That is, record the time element for each individual interference using the same measure, such as minutes or hours or whatever measure you select per day or week. As you will see shortly, this will help you prioritize your interferences in terms of importance and action.  Let’s look at an example.

In this example, suppose you are working in an aviation maintenance, repair and overhaul (MRO) facility and you are responsible for increasing the number of aircraft available on a daily basis.  You haven’t been meeting your contractual targets, so you’ve been receiving large $ penalty losses.  Let’s assume you have already completed a process analysis using a process map or value stream map and have determined the location of your system constraint to be the wait time required to get necessary approvals to permit your maintenance work to begin on the aircraft.  In this scenario you assemble a team of subject matter experts, the people maintaining, repairing and overhauling the aircraft, and you begin construction of an ID.  Since your constraint has been identified as the wait time to begin work on the aircraft, you decide that your goal should be, “Reduced wait time to begin repairs.”   You ask the team the following question, “What is preventing you from beginning work on the aircraft sooner?” One-by-one you then record both the interference the team members have described as well as an “estimate” or “guesstimate” of how much time is being lost for each one.

Figure 2 is an example of the responses (i.e. interferences and estimated times) you received from the team members and the populated boxes surrounding your goal.  If you just eyeball the interferences, you can see that Incorrect Assignments is the largest impediment to reducing the wait time to begin repairs at 210 minutes per week.  This is followed by paperwork and waiting for the rinse crew at 120 minutes each.  Once you feel confident that you have captured the predominant interferences, I recommend that you create either a Pareto Chart or a Pie Chart to prioritize the interferences.


Figure 3 is the Pareto Chart of repair time minutes lost per week as a result of the interferences identified by the team of SME’s.  The Pareto Chart reflects the priority order to “attack” these interferences to authorize repairs to begin sooner on the aircraft.


I mentioned earlier that the ID can be used at different levels, so let me provide an example.  Suppose that one of the major interferences was getting the necessary paperwork to the mechanics.  You assemble the core team again and decide that the goal is to “Reduce Time Waiting for Mechanic’s Paperwork.”  You then ask the team, “What is preventing you from getting the necessary paperwork sooner?”  As described earlier, the team then provides a list of interferences and time estimates that are blocking achievement of this goal.

Figure 4 is an example of this lower level Interference Diagram and as you might expect, there are few interferences listed, but they are much more specific than those in the original ID.  In this example, the team decided if they could come up with a way to stop batching “Daily Cards” the process would improve significantly.  Because of the travel time from the flight line to Maintenance Control (the final location for the Daily Cards) the team recommended purchase of scanners so that a copy of the cards could be emailed to Maintenance Control rather than holding them as batches and transporting them to MC at breaks and lunches.

The Interference Diagram is a thinking process tool that is based more on experience and intuition rather than logic, but it is a tool that has proved to be invaluable in terms of visualizing organization problems at all levels.


Visit our YouTube channel to watch a short video of Bob Sproull presenting an Aviation Maintenance Case Study:


If you would like the full slide set of the case study presentation please contact Morgan Bowman at mbowman@novaces.com

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