Content by Donald J. Wheeler
A History of the Chart for Individual ValuesThe ultimate in homogeneous subgroups
Wed, 06/12/2024 - 12:03
In May 1924, Walter Shewhart wrote a memo that contained the first example of a process behavior chart (i.e., a “control chart”). It was a chart for individual values that would be known today as a p-chart. Shewhart’s insight was that “three-sigma”…
One Technique, Many UsesThe versatility of Shewhart’s charts
Mon, 05/06/2024 - 12:03
One hundred years ago this month, Walter Shewhart wrote a memo that contained the first process behavior chart. In recognition of this centennial, this column reviews four different applications of the techniques that grew out of that memo. The…
The Test to Use Before All Other TestsHow to avoid mistakes in your analysis

Wed, 04/03/2024 - 12:03
When presented with a collection of data from operations or production, many will start their analysis by computing descriptive statistics and fitting a probability model to the data. But before you do this, there’s an easy test that you need to…
What You Need to Know About Weibull DistributionsThe more you know, the easier it becomes to use your data
Wed, 03/06/2024 - 12:03
Over the past two months we’ve considered the properties of lognormal and gamma probability models. Both of these families contain the normal distribution as a limit. To complete our survey of widely used probability models, this column will look at…
What You Need to Know About Gamma Probability ModelsThe more you know, the easier it becomes to use your data
Wed, 02/07/2024 - 12:03
Clear thinking and simplicity of analysis require concise, clear, and correct notions about probability models and how to use them. Here, we’ll examine the basic properties of the family of gamma and chi-square distributions that play major roles in…
What You Need to Know About Lognormal ModelsHow you can filter out noise
Mon, 01/08/2024 - 12:03
When do we need to fit a lognormal distribution to our skewed histograms? This article considers the basic properties of the lognormal family of distributions and reveals some interesting and time-saving characteristics that are useful when…
The Leptokurtophobia PandemicWhat are the symptoms?
Mon, 12/04/2023 - 12:03
Fourteen years ago, I published “Do You Have Leptokurtophobia?” Based on the reaction to that column, the message was needed. In this column, I would like to explain the symptoms of leptokurtophobia and the cure for this pandemic affliction.…
Can We Adjust Our Way to Quality?Part 2: By trying to do better, we can make things worse
Mon, 10/30/2023 - 12:03
In last month’s column, we looked at how process-hyphen-control algorithms work with a process that is subject to occasional upsets. This column will consider how they work with a well-behaved process. Last month we saw that process adjustments can…
Can We Adjust Our Way to Quality?Part 1: Process-hyphen-control illustrated
Mon, 10/02/2023 - 12:03
Many articles and some textbooks describe process behavior charts as a manual technique for keeping a process on target. For example, in Norway the words used for SPC (statistical process control) translate as “statistical process steering.” Here,…
Using the Precision to Tolerance RatioWhat does this ratio tell us?
Tue, 09/05/2023 - 12:03
As we learned last month, the precision to tolerance ratio is a trigonometric function multiplied by a scalar constant. This means that it should never be interpreted as a proportion or percentage. Yet the simple P/T ratio is being used, and…