Excerpt from the Stability Chapter

First get to basic stability
If you have not already been using lean methods and improving processes in all likelihood your individual processes are unstable. Stability is defined as the capability to produce consistent results over time. Instability is the result of variability in your process. It could be that equipment is not well maintained so breaks down regularly. It could be that for any number of reasons defects are regularly produced. Or perhaps there is no standard work and the amount of time it takes to perform a given process varies tremendously from person to person, across shifts, or over time.

The first step in creating lean processes is to achieve a basic level of process stability. The primary objective in developing stable processes is to reach a consistent level of capability. Based on the spiral model of continuous improvement presented in the last chapter, there are increasing degrees of stability. The initial level of stability is generally defined as having the capability to produce consistent results some minimum percentage of the time. This is measured based on the outcome, and is related to producing the same quantity of products, with the same amount of resource time (people and equipment), with a high degree of reliability (the exact level may vary according to the process and conditions, but a reasonable rule of thumb is 80% or better). A simpler indicator would be the ability to meet the customer requirements with quality products the first time through on time (again 80% or better). In many cases the “customer requirement” is not clearly defined and becomes one of the first tasks of the stability phase.

Indicators of instability
There is a wide belief that stability is indicated mainly by equipment performance. This belief causes the pursuit of certain lean tools, namely Quick Changeover, and attacking equipment failures through preventative maintenance as primary activities. Developing process stability is not an end onto itself. In fact, attaining stability is more about creating a foundation for further aspects of a lean process. Through direct observation, an unstable process is indicated by the following conditions:

1. A high degree of variation in performance measures - either pieces produced or pieces per labor hour.
2. Changing the “plan” often when a problem occurs. This includes relocating labor or leaving a position vacant when an absence occurs, moving product to another machine when a breakdown occurs (and thus not producing planned product), and stopping work in the middle of an order to change to another order.
3. It is not possible to observe a consistent pattern or method to the work.
4. Batches or piles of work (WIP) that are random- sometimes more, sometimes less.
5. Sequential operations that operate independently (island processes).
6. Inconsistent or non-existent flow (also indicated by random WIP piles).
7. Frequent use of the following words when describing the operation- “usually, basically, normally, typically, generally, most of the time,” followed by “except when,” as in- “Normally we do this…expect when…. happens, then we do this….” (By its very nature, an unstable operation does not often experience “normal” in terms of consistent method. In fact, the abnormal becomes the normal).
8. Statements such as “We trust the operators to make decisions about how the work is done,” (part of a misguided application of employee empowerment).

It is important to realize that no operation will ever achieve a perfect level of stability, and thus to some degree these conditions will always exist. In fact stability is not only a requirement for flow, but developing flow helps motivate disciplined approaches to stability-they go hand in hand. The main consideration is how unstable the process is, and how stable it needs to become in order to move into the next phase of achieving some degree of flow. Based on the spiral model of continuous improvement, during the incremental leveling phase the operation will be “squeezed” and a higher level of stability will be necessary to meet the tighter requirements. This, in turn, will force a refinement in the methods, beginning a new turn around the spiral in ever tightening cycles.