Monday, March 22, 2010

Six Sigma Tool of the Month-Defect vs Defective

What is a defect? What is a defective? Whats the diference? Why do I care? Last month I discussed capability analysis as the Six Sigma Tool of the Month. Get a refresher on that here. One of the ways that we can measure capability is a statistic called DPMO or Defects Per Million Opportunities. DPMO is a measure of how many times (out of a million) your process would produce good "product" vs bad "product". But that measurement begs the question, What is a defect? So that brings us to today's post.

A defect is a characteristic of the "product" that does not meet the customer's requirements (Specifications). in a DPMO calculation, each one would count as one. Does a defect make a unit of product defective? lets say we are producing a complex product, with multiple quality characteristics, say a new automobile or a monthly sales forecast for 20 products. Does one defect make the car or the sales forecast defective? In other words does a defect affect our decision about the purchase or retention of the product as a whole. If there is a blemish in the paint on the car, should the car go to the junkyard? If one product in our sales forecast is misrepresented by 10% does that mean the whole sales forecast is no good and should be scrapped? Probably not. That's the difference between a defect and a defective. A defective is a unit of product that can not be considered good due to a preponderance of failing quality characteristics (Defects) - OR - a failing quality characteristic that is so vital to the functional purpose of the product that its failing inhibits the primary function of the product. In our new car and sales forecast examples if the car had a paint blemish, a burnt out headlight, a wobbly wheel, a staticy radio, and cracked windshield and tear in the seat. We'd return that car and demand a new one, or not buy it in the first place. In our sales forecast, if one product was off by 10%, and another off by 12% and another off by 23% and so on, eventually we would not believe the forecast at all. Its defective. Again, using our two case examples, if the car was missing the engine or the forecast for a major product was off by 50% or more, that alone would make the forecast defective.

So why care about the difference between a defect and a defective. Its all about the pain the customer is feeling. remember, we measure process capability in terms of the deliverable(s) the customer cares about (specifications). Our reaction to measuring a missing engine in our automobile product as a defect might be different than if we take the customers' perspective that the product is unuseable without the engine (defective).


Anonymous said...

Hi Jim,

I am a student studying six sigma for a university quality management course and I would like to let you know that I have found your blog very helpful in understanding certain concepts.

Keep up the great work!

- Tiffany

Unknown said...

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