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Process Variation

To begin further discussion on how to approach improving histology processes through Lean and Six Sigma methods, we have to first understand key terminology, characteristics, and specific parameters which will define any process. Imagine that we have 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 delivered after noon, this is termed a "defect." Since our customer specification is delivery by noon, and we have delivered items after the delivery time specification, we must identify the late slides as both a process variation and 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 just 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 which impact the outcome of a process or process step. The goal within Six Sigma is to be aware of those fluctutations, what causes them to occur and to greate a consistent process that delivers what the customer expects nearly all the time. Since all processes will have some variation due to natural fluctutions, so every "blip" is not automatically "cause for alarm". Six Sigma defines variations in very specific meanings which have different implications for process performance.
Six Sigma Variation
  1. 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. These are variations that can be addressed by process improvement and process control.
  2. Special cause variation is induced by specific and identifiable causes. Such as slides being late due to the tissue processor not being loaded or started on time. These are usually one-time or at least very infrequent events. Chasing after these fluctuations is " putting out fires" without really improving anything over the long haul.
  3. Defects or Rework occurs when the work is sent back or delayed when it does not meet customer specifications. This could occur 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 special cause variation since this type of variation is defined by its relationship to a standard.
The goal within Six Sigma for process variation is defined specifically and quantified, and always to reduce the variation in the process to a degree that the probability of a particular defect occurring is less than six sigma or 3.4 defects per million opportunity. This is a much clearer target than are defined in other process improvement methods.