The Interference Diagram Part 2

In Part 1 I presented the basic structure of an Interference Diagram (ID).  In this post I will present an actual example of an ID and how to construct one.

Building an Example

With these simple steps in mind let’s use this technique to dissect a situation and determine the interferences.  Let’s assume our example, for the sake of discussion, is using the ID to supplement additional systems thinking that has already occurred.  In this example we’ll assume that you have already conducted a preliminary systems thinking analysis using Goldratt’s Five Focusing steps.  In your analysis you have concluded where the constraint in your system resides. You have completed Step 1 of the Five Focusing Steps (Find the constraint).  You’ve determined that the constraint is a drilling machine that drills some very specific holes, in specific locations, and sometimes the holes are drilled at some odd angles.

There is also an assembly step that takes place at this location.  To drill some of the holes the operator must assemble the part he has, plus an additional part before he can drill the holes.  The additional parts come from another line in your plant.  The output from this machine seems too low.  There is sometimes a blockage of parts in front of it, and starvation for parts behind it.  It appears to have the classic characteristics of a system constraint.  The operator of this machine seems to be busy all the time, and yet the output is low.  Step 2 of the Five Focusing Steps states to “exploit” the constraint – to get more output.  To discover “why” the output is so low you decide to create the interference diagram.   You want to improve the output from this work station, so you ask yourself “What do I want more of?”  The answer is “More machine output.”  So, on a piece o paper or a white board write the following:

So, now you have verbalized what you want more of.  Next, you want to discover what are the interferences or those things that block the operator/machine from getting more machine output?  The first thing you notice is the amount of time the operator spends “Looking for the paperwork!”  We will assume in this scenario that a day is 8 hours in length, so we’ll base our analysis on 8 hours of available time.  You have a quick discussion with the operator to gain more information.  You add the interference to your diagram with the time estimation (some days may be more and some days may be less) of how much time the operator spends every day looking for the correct paperwork.  In this instance looking for paperwork is about 1.5 hours per day.

 Now, ask the question again - “What stops you from getting more of what you want?”  The operator offers an additional interference about how much time he has to spend looking for the additional parts he needs to do the assembly before drilling.  Add this interference to the diagram - “Looking for the parts I need”.  Now, add this as a second interference  with a quantified time estimate.

You ask the operator “are there any others times you have to leave the machine?”  He tells you, “Yes!”  He leaves the machine during lunch and the morning and afternoon breaks.  Lunch is 30 minutes and each break is 15 minutes long, so the total time is 60 minutes.  This time counts as time away from the machine because machine output stops when the operator isn’t there. So add this as interference #3 with the correct time.

Through additional conversations and observations you are able to identify additional items that take time away from machine output.  Add these additional interferences to the diagram with their estimated times.  The figure below is the completed Interference Diagram.

Chances are good that most of these interferences are caused by some type of policy constraint, or procedural constraint.  Another way to enhance the analysis is to paint a picture of how big the interference impact really is.  You can create an impactful picture by transferring this information into an excel format.  You can create a pie chart that paints the visual impact for all of the interferences. The figure below displays the pie chart for this interference analysis.

In my next post, we will discuss how to use the pie chart in this effort.

Bob Sproull