Manufacturing ERP Software
Manufacturing Processes—Production and Business: The Interference Diagram, Part 3

Manufacturing Processes—Production and Business: The Interference Diagram, Part 3

By Bob Sproull

Review of The Interference Diagram, Part 2

In Part 1 of this new series, I introduced two types of Interference Diagrams (ID), provided the steps for constructing an ID, and explained the first two steps.

In today’s post, I will continue with steps 3 and 4 on how to construct an ID and then begin constructing an ID. Much of what I will present in this series of posts is taken from one of my books, [1] Epiphanized—A Novel on Unifying Theory of Constraints, Lean, and Six Sigma, co-authored with Bruce Nelson.

Steps 3 and 4 in constructing an Interference Diagram

As you will recall from Part 1 of this series, our example is a machine in a production line identified as the constraint. We have defined the objective and the interferences and now we are moving on to the next steps in this four-step process.

Step 3 - Quantify the time component for ALL interferences

Quantifying the time component associated with the interferences is a vital step in fully appreciating the impact of the interference on the available time. The time component will help filter the important few interferences from the (potentially) many trivial interferences. It will help to emphasize those items with the greatest impact when we communicate our findings. Knowing the impact of the time component will also be useful in determining a sequence of interferences to eliminate.

Some of the interferences will be more important than others; they are not all equal. When you accurately quantify the time component for the interference, it also allows for excellent Pareto analysis. As a refresher, Pareto Analysis is a statistical technique in decision-making used for the selection of a limited number of tasks that produce significant overall effect. It uses the Pareto Principle (also known as the 80/20 rule) the idea that by doing 20% of the work you can generate 80% of the benefit of doing the entire job. Pareto analysis will align the interference, based on time distribution, and determine which interference is most impactful. It will also help to determine the intrinsic order of improvement. Pareto also allows the focus necessary to gain the most leverage from the action implemented.

It is also not realistic to assume that ALL interferences can be reduced and/or eliminated. Some interferences that do not offer themselves as candidates for elimination. They are candidates for a reduced time impact. In other words, if an interference with a time impact of 45 minutes can be reduced to 15 minutes, then the benefit gained for the system is 30 minutes more time for the constraint to produce. In other cases, the interferences cannot be reduced or eliminated at all. In our example, the time for breaks and lunch cannot be removed. Employees are allowed lunch and break time. However, as an alternative, you could gain some machine time by having an alternate person or crew to work the machine during lunch and breaks.

What you are really looking for in Step 3 is to quantify in time, those activities that are stealing time away from your constraint operation. If you can eliminate, reduce, or off-load some of these activities, then more time is available to get more parts through the constraint. If you know the interferences and correct them, this should result in more output from the machine and more throughput through the system.

Step 4 – Develop alternatives to the interferences

The list of obstacles/interferences defines all of the elements that stand in the way of having more of what you want. If these did not exist, then achieving the goal would be easier. With the interferences defined, you should be able to counter their seemingly negative effects with an Injection/Intermediate Objective (IO). This can be accomplished by asking the question: “What must be done so that the interference no longer exists?” Whatever your answer might be is the IO to overcome the negative impact of the interference.

Continue working your way down the list and create an Injection/Intermediate Objective for each Inference/Obstacle listed. The items on IO list are the things that must be accomplished to reduce or eliminate the negative effects of the interferences. If they cannot achieve this, then consider revising your IO list until the answers are sufficient to remove the interferences. With the addition of IO, the list should provide sufficient ideas to move you closer to the objective you have established. With the completed list, you have now precisely defined “what to change to.”

Obstacles/Interferences Intermediate Objective/Injections

1. Parts not available to work.

1. Parts are kitted and ready for use.

2. Operator in on break/lunch.

2. Train an alternate crew or person.

3. Operator has to find his own parts.

3. Parts delivered to operator.

4. Operator is looking for the Supervisor.

4. Supervisor notification system.

5. Operator is looking for paper work.

5. Paperwork follows job in the system.

6. Machine is broken.

6. Preventive maintenance (priority #1).

Figure 1 shows what a completed Interference Diagram might look like for our example. The inner circle contains the objective or goal, with each of the obstacles/interferences listed around the inner-circle. The direction of the arrows makes no difference because these arrows are based on the ID user’s intuition and not necessity or sufficiency logic.

Figure 1: Basic Layout of Interference Diagram

interference diagram

Coming in the next post

In the next post, I will continue this series on the Interference Diagram (ID) and construct our ID using the information collected in our last post’s case study. As always, if you have any questions or comments about any of my posts, leave a message and I will respond.

Until next time.

Bob Sproull


[1] Epiphanized – A Novel on Unifying Theory of Constraints, Lean, and Six Sigma, 2nd edition, Bob Sproull and Bruce Nelson, CRC Press, 2015.

Bob Sproull

About the author

Bob Sproull has helped businesses across the manufacturing spectrum improve their operations for more than 40 years.

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