Statistical Thinking vs. Engineering Thinking

Statistical Thinking vs. Engineering Thinking

Credit: Wikimedia Commons

Thinking of the core concepts of quality and statistical process control we consider statistical thinking vs. engineering thinking.

Engineers have it made. In the world of engineering, the formulas are precise and definitive. When products are designed, the dimensions are exact, the calculations are accurate, and the resulting characteristics are known.

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Can You Make Two Things EXACTLY Alike?

 

Variation and controlling variability.

Why do We Need Tolerances?

Specs and Work QuotasIf it was possible to make all items alike, we would not need tolerances. But no two things are exactly alike. So, we must make allowances for the differences. Tolerances are these allowances. They provide a range over which a product can vary and still be accepted by the customer.

Of course, “accepted” is not “desired,” and in the battle over width of tolerances the idea of a specific target has been lost. For example, ask almost any machinist what size a part is supposed to be, and you will get a range as the answer — “from 2.048 to 2.052 inches.” Why isn’t the answer “2.050 inches” if that is the target? Because with tolerances, we tend to aim for the acceptable range instead of the exact target. This means we are less likely to hit the target. It also means our variance will most likely be greater than if we aimed for the target.

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That leaves us with a bit of a dilemma.

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How Quality Affects Your Customer’s Buying Choice

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In the previous posts in this series, we gained an understanding of what the quality means and that it has a different meaning to different parties.  We then acknowledged that the way the customer defines quality, while highly subjective and of utmost concern, can be simplified.  Today, we walk a step further, considering how a producer’s view on quality, while technically closer to optimal, may not be the best for business.  This variance in process and results is how quality affects your customer’s buying choice.  Let explore this further.

The illustration below shows the output of two companies making the same part. Company A has much tighter control over the dimension but their process is centered near the low limit. Company B, however, doesn’t have enough control to keep their products within spec all the time and produces a 3% out-of-spec product.

FIgure 1-1 The Book of Statistical Process Control

From a producer’s point of view, Company A would appear to be the better company. The tighter control of the dimensions reduces the cost of materials.


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Company A has much tighter control over the dimension but their process is centered near the low limit. Company B, however, doesn’t have enough control to keep their products within spec all the time and produces a 3% out-of-spec product. Again, from a producer’s point of view, Company A would appear to be the better company. The tighter control of the dimensions reduces the cost of materials.

But this is only one way to look at it.  What about the customer’s point-of-view?

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How Does Your Customer (Not You) Define Quality?

[This content is from Zontec’s The Book of Statistical Process Control.  You can download a free copy here.]

In a separate post on quality itself, we defined quality.  This is not an easy task since quality, like beauty, is in the eye of the beholder. We identified five points we can all agree on that equate to quality.  More important than this though is how your customer (not you) defines quality. When it comes to quality, your customer is concerned with three different areas: design quality, manufacturing quality, and performance quality.

Rails

Credit: flickr: Particularly Everything

Design Quality

Design Quality is the intended shape, size, color and function of the product. The designer must assess the needs of the customer and select characteristic values that will create the greatest customer satisfaction. 

Manufacturing Quality

Manufacturing Quality is how well the product meets the design. It is this type of quality that is the main subject of our free e-Book, The Book of Statistical Process Control. The better the manufacturing quality is, the better the product quality will be. And if this type of quality is good, product quality will be more consistent. Keep in mind, however, that while improving manufacturing will improve a product to a certain extent, it will never improve a poor design.

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What is Quality? Quality Defined

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What is Quality?

Credit: flickr: Particularly Everything

In the 35+ years since our founding, we’ve detailed the concepts of quality and SPC to a great degree. However, it’s important from time to time to remind ourselves just what is quality and why is it important.

Before we can control quality, we need to understand what the word “quality” means. Quality, like beauty, is in the eye of the beholder. Each of us judges the quality of many items every day, and we each have a different idea about what quality is for each item. We know what we like or what we want, and what we don’t like or want. How we determine the quality of an item depends on both our personal preference and our frame of reference for that particular item. Read more