Variation

To fully understand quality, and know how to improve quality, it is necessary to understand variation.  Variation is always present and is classified in to two types; common, and special.

Before describing the two types of variation we will discuss variation itself.  Usually in the early years of education students learn that no two things are really exactly the same.  It may appear so to the human view, or even be measured the same, but if we have a way to view or measure more accurately there is always a difference.   Quality, more than any other single aspect is being able to make each part like the last one B that is ensure part to part consistency.  Or in the case of processes, to have the process run the exactly the same each time it operates.  Consistency, making or doing things the exact same each time, or at least exactly as it was intended, is the primary factor that impacts quality. 

Therefore the primary reason for the problems that occur in accomplishing quality is because of variation.  Because variation happens all the time, and because it is what we don=t want to have happen to accomplish quality we must understand variation, why and how it occurs and how to eliminate it if possible and if not then at least reduce it.

Here is an example of the occurrence of variation. The chart shows the variation in the diameter of 10 supposedly identical pieces of steel. 

Notice that the diameter varies from part to part.  What are some possible causes of this variation?

Check which factors you think could cause this variation:

Measurement error                         Tool wear on machine

Time of day that parts were made    Operator of machine that made parts

Loose screw on machine                  Other