Variation Amplification: Even a 3-Year-Old Understands It...Do You?

This weekend my 3-year-old son and I were playing with his marble run set, and he said to me, "The marbles start together, but they don't finish together!"

It dawned on me that the phenomenon he was observing seems so obvious in the context of a marble run, and yet many practitioners fail to see the same thing happening in their processes.  I quickly made a video of me placing six marbles in simultaneously so I could illustrate to others what I will call "variation amplification:"

It is obvious in the video that there is little variation in the positions of the marbles in the beginning, but as they progress through the run the variation in times becomes larger and larger.  In fact, these facts are obvious even to a 3-year-old:

  • The balls spread out as they progress
  • Certain parts of the run cause the balls to spread out more than others
  • The balls do not finish in the same order that they started
  • Some pieces allow balls to change position while others do not

To help further illustrate some of these points, here is a graph (created in Minitab Statistical Software) of each of the six balls at various points in the run:

Time versus Position

At this point, these facts all seem very obvious.  But when working to improve cycle times of a process—whether through lean efforts, a kaizen event, or a Six Sigma project—many practitioners completely fail to take advantage of these characteristics. 

Some will even tell you that times "even out" during the process, and a part that took an exceptionally long time in one step of the process will probably take a short time on another so that parts end up with roughly similar total cycle times. 

In reality, that part is just as likely to take exceptionally long again on another step and be even further from average. This is the essence of variation amplification: variance in cycle times will only increase at each step of the process and, without some control in place, will never decrease.

Consider processes of invoice payments in a finance department—or indeed most other transactional processes, whether in an office, healthcare, or other environment. The points from above can be generalized to:

  • "Parts" starting at the same time will spread apart from one another as they progress through the process, and will not finish together.
  • Certain steps in the process will cause more spread than others.
  • Parts will not complete each step in the same order that they completed the previous step.
  • Only some steps will allow for re-ordering.

So how do we combat variation amplification in transactional processes?  There are multiple lean tools at our disposal. I won't pretend that a few sentences in a blog post can cover everything, but I will offer a few starting points.

  1. Collect data to find out which steps are adding the most to the variation. In the marble run it is obvious that the round "bowls" are the biggest contributors, but in most transactional processes, various steps are electronic and it is difficult to watch a part progress through the process.  Collect data to gain clarity.  Then focus on the biggest contributor(s) first.
  2. In most cases, reducing the average time in a step will also reduce the variation.
  3. Establish flow so that parts are not re-ordering (FIFO).
  4. Allow parts to queue prior to steps that add significant variation. As you reduce the cycle time and variation within that step you can reduce the queue until (hopefully) you establish a pull system, where there is little or no need for queuing.

From a simple marble game a 3-year-old understood variation amplification, and you likely could too when you watched the video.  But can you see that the same phenomenon is happening in transactional processes all around you?

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