Six Sigma SPC - Statistical Process Control

100% Inspection Does It Cost Or Pay?
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These articles are from the Six Sigma SPC Newsletter and other publications. All articles written by Jim Winings

Feb. 2000

This is an article that original appeared in my newsletter and then was re-edited and cleaned up and appeared in the Feb. 2001 Issue of Six Sigma Exchange Newsletter. This is the unedited version of the one that appeared in the Six Sigma Exchange Newsletter.


Sometimes there is no choice. If you must get product out the door and all you have in stock is rejected or defective material, then you must sort through it and try to find the good ones. (I would also look for another supplier or two) This is very costly. (I would be trying to get the supplier to pick up your cost of doing their job) But, what choice is there? But you MUST meet customer's requirements.

But first a little more on 100% inspection. I am a firm believer that YOU CAN NOT INSPECT QUALITY INTO A PRODUCT. 100% Inspection has an effectiveness of 40-65%. And that doesn't count the 5-10% breakage. It is a waste of people power. (Formally known as man-power) I remember reading, I think it was in one of Juran's books that 100% inspection is only 60% effective. But I have found it to be a little less than that.

According to Dr. Jim Stewart of Northern Illinois University in DeKalb, IL

While working on my dissertation, I was reviewing some trade magazines from the 50's. There were a number of case studies showing 50-75% efficiency and a breakage rate (visual inspections of wire wraps with pics) of 10-15%. Giving an effectiveness of 40-65%

It was just about a year before the 'Six Sigma' era. Motorola had just came out with a couple of new radios. One was a KDT, a data terminal using 900Mhz, IBM was the pilot customer. The other was the STX 800Mhz two-way trunking radio and it was being sold as a pilot project to the City Of Miami's Police Department.

The quality of the KDT's was so bad that rumor had it there were two IBM engineers testing each unit themselves before authorizing shipments. The STX radios had received several complains that the dispatchers could not hear the police officers in the field. So the development and process engineers got to work. But everything seemed to be fine on that end. Ergo, it MUST be the labor force.

Motorola had released two new products, both from what at that time was the 'Communication Sector' now called 'Land Mobile' and before they got into full production there were a gross amount of customer complaints.

I had already gone through, what at that time was the ASQC courses, where, as I recall, they said that 100% inspection is only 60% effective, the quality manager decided that we would 100% inspect the radios until we got 'x' good ones, then the sample number would decrease. Well I had the proverbial cow. I started my shift, being the only engineering support person on 2nd shift, I thought, if I have to look at every radio, then I'm going to collect data so that it won't be a waste of time. The radio factory only collected data on the prototypes and maybe the first run. The Quality Engineering gurus were glad to see some data finally being taken on actual product being shipped.

At that time, Motorola tested for a test called 'Dev' by modulating the radio with a 1khz tone. I thought to myself, this is silly, who talks at 1khz. (Common Sense) Since we were just whistling in to the radio and then checking the Motorola service monitor anyway, I started whistling not a single tone, but a sweeping tone and taking my data. If a reading was a little low, I would try around that frequency to see if I could produce a failure, usually caused be an external failure. At the end of the shift, I went to the office, and on a Compaq 'portable', (you know, that white Comapq with the green screen that looked like a sewing machine when closed up and someone had to guts to call a 'portable'), and with SQCpak entered my data. I then printed distribution, X-Bar and Range control charts and left them on my boss's desk. The next day he went to the daily product meeting and presented the Development Engineering Manager with my charts.

Fortunately the Development Engineering Manager actually looked at the charts and noticed that consistently, my 'Dev' numbers where better than productions and what they saw in the prototype. Production had an automatic testing machine to test the radios, and the quality department tested the radios by hand. The day after that, the Development Engineering Manager waited for me to come in and gave me some data the development team had collected to put into the SPC, (statistical process control), software. The results were exactly what they had gotten before. The very next day development engineering watched me to find out what I was doing different. It was of course the sweeping frequencies. (i.e. more 'real world' like)

This better 'Dev' number allowed them to change a part in the radio that fixed the problem with the STX radio. Now, or at least as of my last day there, Motorola was testing all new radios with this sweeping frequency. What's my point, we could have 100% inspected radios for years and never found the problem. We were looking in the wrong place. So if you find yourself needing to 100% inspect, at least collect data and do the control charts, even just X Bar, Range and a Distribution charts can tell you a lot. You never know what it may reveal until you do.

Shortly after that I was promoted to Component Engineering where I first heard of Six Sigma. I wrote software that allowed Motorola and their suppliers to use a standard SPC package to aid in starting their Six Sigma program. This allowed suppliers to submit sample data and control charts with every shipment coming into VQA. Without this software, it would have been a mess trying to train the inspectors what to look for with the different SPC charts and without training the inspectors, the Quality Engineers would have to look at each shipment and that would have just made them over priced inspectors.

The concepts of Six Sigma are much larger than just measurements of course. It's also about getting a Return On Net Investments, (RONI), and Return On Net Assets, (RONA).

It's About Working Smarter, Not Harder.

It's About Common Sense.

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