Have you been on a project where workers are standing around with nothing to do or areas on your project ready to go with nobody around and all you hear are crickets chirping? This is caused by variation in production which simply means you have inconsistent processes, and you will always end up with inconsistent results.
Variation is inevitable. It’s caused by fluctuations in the process and set by predetermined standards. Deviation or fluctuations in the performance of a crew will cause variation and, ultimately, corrupt reliable workflow. The result is workers waiting for work or work waiting for workers, the two biggest forms of waste on a project. One of the fundamental principles of Lean Construction is reducing variation on construction sites.
To help understand variation, we need to know the types commonly found on projects. Common cause (random) variation is caused by the property of the process or procedure. Some examples include:
- worker motivation
- safe work practices
- worker experience
- following inconsistent processes or procedures
Special cause (non-random) variation is caused by external influences to the process. Some examples include:
- environmental conditions
- equipment breaking down
- material delivery interruptions
- delays in submittals
- design errors
Variation of either kind is bad on the production system. It can cause a large amount of strain on the trades within a system. The trades are either constantly underused, overused or alternating between the two. The goal is to reduce the effect of variation on the system. There has to be a way to reduce the variation. Once this happens, you begin to see better workflow on the project.
Workflow is the progression of work within a trade or from one trade to another. You can have either predictable or reliable workflow. Predictable and reliable are not the same thing. Predictable is the ability to consistently foretell something. Reliable is the long-term consistency of a system. Knowing that workflow is reliable indicates that we can have predictable starts and release of work. The Last Planner® System was developed to increase the reliability of workflow.
So what can we do to minimize the effects of variation on our projects? How can we make our work more reliable? What is the Lean strategy to do this? Let’s take a look at a three-step process for minimizing, reducing and removing variation.
First step is to use buffers. Buffers simply reduce the impact of variation. There are three specific types:
- Size or inventory
- Capacity
- Schedule
Size or inventory buffers are created to keep excess materials on or near the site. This helps reduce the impact on the variation in the supply chain. Capacity buffers create room to absorb periods of excess demand. This is not practiced widely in construction, but better than inventory buffers. And finally, schedule buffers, also known as plan buffers or workable backlog. This is work not on the critical path that acts as a backup for planned work not able to be performed for some reason. Simply a “Plan B”. This is frequently used on Lean construction projects. It must be recognized that buffers are not long-term solutions and should be used cautiously. They are not solutions to the problem and should be continuously reduced.
Second step is work on reducing and removing variation. Experimentation and continuous improvement are needed to develop ways to mitigate variation and achieve reliable workflow. One of the eight recognized wastes is underuse of resources/talents. We have to tap into our workforce’s expertise and experience to make improvements on how work is performed. We need to pool the collective knowledge of the individuals involved in a process or operation to find alternative solutions. This is why we do pull planning sessions with the last planners on the project, to come up with the best plan on how to execute the work. It’s all about collaboration.
Third step is to lower the river. Problems below the surface become exposed that you wouldn’t see otherwise. This allows us to identify system problems and try to start eliminating them. An example of this would be to gradually reduce the stock of materials on site and see what happens to your production process and pay attention to new areas for improvement.
Variation is real and needs to be a point of focus to realize the corruption it puts on a production system. Once you begin to reduce variation, real flow starts to happen. Once you have flow, then you will see a lot less of workers waiting on work and work waiting on workers. Real value!
References
Content from this blog was derived from an AGC LCEP course entitled: “Variation in Production Systems”. To find more information on this subject as well as other units available, visit www.agc.org/LCEP or www.agcleanforum.org.
