SPC Guide
First Word


Gregory Ferguson

Everyday SPC

Basic principles of SPC are best explained using examples we can all relate to.

It seems to be an unwavering aspect of human learning that it's easier to understand something in a familiar environment. For example, a friend of mine has done a great deal of training in SPC. He was once teaching a group of people who worked in a factory that produced labels for one of the nation's largest label makers. He was giving them an example of how to use a control chart. In the example he said that the factory was producing widgets. (The word "widget" is often used to represent some unspecified product.) The students were very confused and kept asking him to clarify. After several unsuccessful attempts to explain his point, my friend went back and gave the same example, only this time he said the factory was producing labels. Now they got it. The simple device of using a familiar example helped them understand.With this in mind, I've been trying for some time to think of an example of an SPC study that would be familiar to my readers.

 I realize that not everyone works in a manufacturing facility, so here is an example you can try on your own. I got the idea when my son, Michael, turned 16. Anyone who's taught a teenager to drive knows it can be difficult. Up until Michael started driving, my old Toyota 4x4 was a pretty good truck. I had owned it for 10 years and I knew it so well I could tell when the oil was dirty by the way it sounded. Michael was a special cause of variation. He made a dramatic change in the mean time to failure of the vehicle. After a few months of his involvement, my delicate old truck gave up the ghost, and I ended up buying a new one. It offered several advantages over the old truck. For one thing, it would start. For another, it got great gas mileage.

 After some time, I noticed that its gas mileage had declined. I have certain modest ambitions as an off-road driver, and at first I thought my gas mileage might have been adversely affected by those long grinding runs down dry riverbeds. I guessed that spinning the tires while I wasn't moving might degrade my miles per gallon. I decided to test my theory.

 And that's where SPC comes in. SPC is really just a set of tools that lets you test theories. My theory was that I got very bad mileage off-road but that my journeys on velvet-smooth pavement were as fuel-efficient as ever. I started keeping track of my mileage, resetting the trip odometer every time I filled up with gas.

 As is the case in many SPC studies, I soon learned that my data-collection activities would have to be refined. Checking the mileage after each weekly refueling wasn't good enough, so I had to improve my measurement system. I began to keep track of my mileage after each quarter-tank of gas. This gave me quicker feedback on how I was doing. It also taught me rather quickly that my gas gauge is nonlinear, as the first quarter of the gauge shows the truck getting considerably more miles to the gallon than the second.

 I decided to fill up right after one of my off-road excursions and stay on pavement until the tank was empty. This would give me a baseline for driving on good roads. I pulled into the first gas station I could find after I returned to civilization and tried to fill up my tank. Again--as frequently happens in SPC studies--I ran into trouble. The gas station was very busy and the only open pump didn't work right. After every few ounces of gas flowed into my tank, the pump would shut itself off. I kept squeezing the stupid handle until the tank was full and was soon rewarded with evidence that my theory was correct: My gas mileage was over the top. I drove farther on that tank of gas than ever before. I was content.

 However, the world has a habit of clouting overconfident fools in the head. I soon learned that my theory was wrong. On my next tank of gas, my mileage plummeted. It's at this stage in a study that you want to slap, scratch and scream a bit just to let the gods of experimentation know that you really aren't having fun.

 Still, SPC is an iterative process. You develop a theory, and then you collect data. If the data doesn't support your theory, you have to revise it and then collect more data. Sometimes this process goes on for months.

 The next time I filled up, the pump shut off right after I started to fill the tank. For an agonizing moment I thought I had another bad pump. But then I remembered that the best mileage I ever got was right after I had filled up with the defective pump; maybe the pump had affected my gas mileage. After I had put in the customary 16 gallons, I gave the pump another shot. I found that by giving it a few ounces at a time, I could put another two gallons in my tank. Topping off like this isn't an especially good idea, but for the purposes of my study, it was a breakthrough. I had been computing the miles per gallon based on the amount of gas I added at fill-up. I had assumed that a 'full tank' was a constant, but it wasn't. The amount of gas it took to fill my tank was an important variable and had a dramatic effect on my mileage, a special cause of variation.

 The object of an SPC study is to learn how the process works. I learned that my gas gauge was nonlinear, that my driving habits affect my mileage and that my feed tube can hold an extra two gallons of gas. That's quite a bit to discover about something you thought you already understood.

 So how about it, fans? Can you do a study of your gas mileage? I'll bet you'll learn a lot about your vehicle and about SPC. Let me know how you make out.


About the author

 Gregory P. Ferguson is senior quality engineer at Global Solar Energy in Tucson, Arizona. Comments, questions and suggestions can be e-mailed to him at .

Menu Level Above 

This Menu LeveL 

Menu  Level Below 

[Contents] [News] [WebLinks] [Columnists]
[Applications] [Software] [SPC Guide] [Letters] [First Word] [Books]

Copyright 2000 QCI International. All rights reserved.
Quality Digest can be reached by phone at (530) 893-4095. E-mail:
Click Here

Today's Specials