All Features
Patrick Runkel
It’s usually not a good idea to rely solely on a single statistic to draw conclusions about your process. Do that, and you could fall into the clutches of the “duck-rabbit” illusion shown below.
If you fix your eyes solely on the duck, you’ll miss the rabbit—and vice-versa.
If you’re using…
John Niggl
Ever wondered why quality control (QC) professionals check a sample instead of 100 percent of a shipment during inspection? Or maybe you’ve wondered why they use acceptance sampling, rather than simply inspecting an arbitrary quantity of goods, such as 10 or 20 percent?
Most importers value the…
Donald J. Wheeler
Last month I mentioned that we can put autocorrelated data on a process behavior chart. But what is autocorrelated data and what does it tell us about our processes? This article will use examples to answer both of these questions.
Autocorrelation (aka serial correlation) describes how the values…
Davis Balestracci
Recently I demonstrated a common incorrect technique for comparing percentage rate performances—based of course in the usual normal distribution nonsense. Let’s revisit those data with a superior alternative.
To quickly review the scenario: In an effort to reduce unnecessary expensive…
Donald J. Wheeler
The simplest type of process behavior chart is the chart for individual values and a moving range. It allows us to plot a point every time we get a value, making it perfect for data that occur one value at a time. A brief history of this simple chart follows.
In the 1920s, Walter A. Shewhart…
Steve Daum
I have daily conversations with manufacturer plant managers, quality managers, engineers, supervisors, and plant production workers about challenges when using statistical process control (SPC). Of the mistakes I witness in the application of SPC, I’d like to share the five most prevalent; they…
Davis Balestracci
My last column mentioned how doctors and hospitals are currently being victimized with draconian reactions to rankings, either interpreted literally or filtered through the results of some type of statistical analysis. Besides the potential serious financial consequences of using rankings in the…
Derek Benson
How early is too early to introduce quality into your everyday life? Have we missed out on improvement opportunities in our personal lives along our paths to achieving our career goals as quality professionals? These questions have me pondering how life could have been different for me growing up…
Donald J. Wheeler
In their recent article, “We Do Need Good Measurements,” Professors Stefan H. Steiner and R. Jock MacKay take exception to two of my Quality Digest articles, “Don’t We Need Good Measurements?” and “The Intraclass Correlation Coefficient.” While we all want good measurements, the trick is in…
Stefan H. Steiner, R. Jock MacKay
In his February 2017 Quality Digest column, “Don’t We Need Good Measurements?” Donald J. Wheeler recommends that a measurement system contributing up to 80 percent of the overall variation (on the variance scale) is good enough to detect persistent mean shifts when using a process behavior (…
Joel Smith
In parts one and two of “Gauging Gage,” we looked at the numbers of parts, operators, and replicates used in a gage repeatability and reproducibility (GR&R) study and how accurately we could estimate %Contribution based on the choice for each. In doing so, I hoped to provide you with valuable…
Joel Smith
In part one of “Gauging Gage,” I looked at how adequate a sampling of 10 parts is for a gage repeatability and reproducibility (GR&R) study and provided some advice based on the results.
Now I want to turn my attention to the other two factors in the standard gage experiment: three operators…
Davis Balestracci
Don’t tell me you’re not tempted to look when you spot a magazine cover saying, “How does your state rank in [trendy topic du jour]?” Many of these alleged analyses rank groups on several factors, then compare the groups’ sum totals of their respective ranks to make conclusions.
For example, in…
Joel Smith
‘You take 10 parts and have three operators measure each two times.”
This standard approach to a gage repeatability and reproducibility (GR&R) experiment is so common, so accepted, so ubiquitous, that few people ever question whether it is effective. Obviously, one could look at whether three…
Fred Faltin
All of us draw conclusions based on what we see happening around us. Often what we’re observing is a sample from some larger population of events, and we draw inferences based on the sample without even realizing it. If the sample we observe is not a representative one, our resulting judgments can…
Donald J. Wheeler
Here we take a serious look at some nonsensical ideas about capability ratios. Following a quick review of predictability and capability and a brief discussion of the traditional ways of characterizing capability and performance, we will consider the shortcomings of four bits of capability…
Steve Moore
When I entered college in the fall of 1970, I had a nice slide rule (or “slipstick” as some of us called it) that I proudly carried in a leather case to my engineering and chemistry classes. Virtually everyone at North Carolina State University had a slide rule then, but by the time I was a senior…
Davis Balestracci
Many talk about reducing variation to improve quality. Does that include human variation, where everyone takes a different approach to improving overall improvement processes? What would happen if this variation were reduced?
Would some of you lean folks be interested in spearheading an effort to…
Quality Transformation With David Schwinn
This month’s column comes from a convergence of finishing my article, “Statistical Thinking for OD Professionals,” for the OD Practitioner, and reading “How Statistics Lost their Power—and Why We Should Fear What Comes Next” in the Guardian, and Weapons of Math Destruction by Cathy O’Neil (Crown…
Donald J. Wheeler
A recent question from a statistician in Germany led me to the realization that the F-test of analysis of variance (ANOVA) fame is in serious need of an update.
What the F-ratio does
The F-ratio, created by Sir Ronald Fisher around 1925, is a generalization of Student’s t-test for comparing two…
Patrick Runkel
Did you ever wonder why statistical analyses and concepts often have such weird, cryptic names?
One conspiracy theory points to the workings of a secret committee called the ICSSNN. The International Committee for Sadistic Statistical Nomenclature and Numerophobia was formed solely to befuddle…
Davis Balestracci
According to Mark Graham Brown, from his book Keeping Score (Productivity Press, 2006), 50 percent of the time leaders spend in meetings involving data is waste, 80 percent of the pounds of published financial data is waste, 60 percent of the pounds of published operational data is waste, and…
Donald J. Wheeler
The average and range chart handles most situations where the data can be logically organized into homogeneous subgroups. However, this chart breaks down when faced with a hierarchical data structure containing two or more levels of routine variation. For these situations I developed the three-way…
Bruno Scibilia
Genichi Taguchi is famous for his pioneering methods of robust quality engineering. One of the major contributions that he made to quality improvement methods is Taguchi designs.
Designed experiments were first used by agronomists during the last century. This method seemed highly theoretical at…
Davis Balestracci
Just curious: Do you have monthly (and/or quarterly and/or even weekly) “How’re we doin’?” meetings like the end-of-year scenario described in my November and December columns last year—about budgets, financials, never events, incidents, near misses, machine downtime, productivity, root cause…
