To begin a further discussion on improving histology processes through Lean and Six Sigma methods, we first have to understand key terminology, characteristics, and specific parameters that will define any process. Imagine a histology laboratory that expects all hematoxylin and eosin (H&E) slides to be delivered by noon each day to the pathologist. If the slides are offered in the afternoon, this is termed a "defect." Since the customer specification is delivered by noon, and there are delivered items after the delivery time specification, the late slides must be identified as both a process variation and a process defect. This is true, even though there may not be any defects or errors within the slides themselves.
Variation in Six Sigma is any result that is not what the customer expects, even if it is not when the customer has expected it. Variation can also include just the random fluctuations within the process itself, which may not result in defects, as well as variations that impact the outcome of a process or process step. The goal of Six Sigma is to be aware of those fluctuations and what causes them to occur and to create a consistent process that delivers what the customer expects nearly all the time. Since all processes will have some variation due to natural fluctuations, every "blip" is not automatically "cause for alarm." Six Sigma defines variations in specific meanings with different implications for process performance.
Six Sigma Variation
- The natural variation or "common cause" variation is the natural fluctuations in process flow introduced by individuals, slight differences in execution, or instrument performance fluctuations. Process improvement and control can address these variations.
- Specific and identifiable causes induce particular cause variation. An example is slides late due to the tissue processor not loading or starting on time. These are usually one-time or at least very infrequent events. Chasing after these fluctuations is " putting out fires" without improving anything over the long haul.
- Defects or rework occurs when the work is sent back or delayed when it does not meet customer specifications. This could happen, for example, when the tissue section quality is poor, and the Pathologist must request a re-cut section of better quality. Defect variation is similar to other forms of variation in that the customer does not get what they want or expect. However, note that defect variation differs from the common cause and particular cause variation since this type of variation is defined by its relationship to a standard.
The goal within Six Sigma for process variation is explicitly defined and quantified. It is always to reduce the variation in the process to the degree that the probability of a particular defect occurring is less than six Sigma or 3.4 defects per million opportunities. This is a much more precise target than other process improvement methods.
