Content by Donald J. Wheeler
Analyzing Observational DataShewhart’s economic operation is the starting point for lean production
Mon, 02/07/2022 - 12:03
Most of the world’s data are obtained as byproducts of operations. These observational data track what happens over time and have a structure that requires a different approach to analysis than that used for experimental data. An understanding of…
How Can the Sum of Skewed Variables Be Normally Distributed?One of the fundamental mysteries of probability theory
Mon, 01/03/2022 - 12:03
On the face of it, it seems to be impossible for skewed variables to add up to a normally distributed result. Yet both common experience and mathematical theory combine to show us that this does indeed happen. In fact it is a fundamental property of…
Torturing the DataWhen is a prediction more than just wishful thinking?
Mon, 12/13/2021 - 12:03
Management requires prediction. However, when making predictions it is easy to torture the data until they surrender and tell you what you expect to hear. Even though this torture may be unintentional, it can keep you from hearing the story the data…
So What Is the Fraction Nonconforming?How to know what the data are really telling you
Mon, 11/08/2021 - 12:03
One of the most common questions about any production process is, “What is the fraction nonconforming?” Many different approaches have been used to answer this question. This article will compare the two most widely used approaches and define the…
So You Want to Use a p-Chart?Not all count-based data will qualify
Mon, 10/04/2021 - 12:03
First, let it be known that all charts for count-based data are charts for individual values. Regardless of whether we are working with a count or a rate, we obtain one value per time period and want to plot a point every time we get a value. This…
So What Are Skewness and Kurtosis?And what do they add to the story?
Tue, 09/07/2021 - 12:03
What do the shape statistics known as skewness and kurtosis tell us about our data? Last month we saw how the average and standard deviation define the balance point and radius of gyration for our data. Once we have these two quantities the…
What Are the Variance and Standard Deviation?And what do they tell us about our histogram?
Mon, 08/02/2021 - 12:03
Your software routinely gives you four descriptive statistics for your data: the average, the standard deviation, the skewness, and the kurtosis. Of these only the average is easy to understand. This article and the next illustrate what these…
Problems With the AIAG Gauge R&R StudyAnd how to fix them so as to avoid the nonsense
Tue, 07/06/2021 - 12:03
Measurement error is ubiquitous. As a result, over the past 250 years, different areas of science and engineering have come up with many different ways to deal with the problem of measurement error. One approach to the problem of measurement error…
More About the Precision to Tolerance RatioWhat does it tell us about the usefulness of a measurement?
Tue, 06/01/2021 - 12:03
For more than 40 years it has been common to use the precision to tolerance ratio (P/T ratio) to compare the standard deviation of measurement error with the specified tolerance for a particular product. The purpose of this comparison being an…
When to Use Tightened SpecificationsHow to ship conforming product
Mon, 05/03/2021 - 12:03
In my article, “Tightened 100% Inspection” (Quality Digest, March 29, 2021), we found that the excess costs associated with tightened specification limits are generally prohibitive. Here we consider the question: “Under what conditions can we use…