Content by Donald J. Wheeler
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…
Questions About Type 1 Repeatability StudiesHow to avoid some pitfalls
Mon, 08/07/2023 - 12:04
A simple approach for quantifying measurement error that has been around for over 200 years has recently been packaged as a “Type 1 repeatability study.” This column considers various questions surrounding this technique. A Type 1 repeatability…
The Problem of Chunky DataWhat happens when the measurement increment gets too large?
Mon, 07/10/2023 - 12:03
Chunky data can distort your computations and result in an erroneous interpretation of your data. This column explains the signs of chunky data, outlines the nature of the problem that causes it, and suggests what to do when it occurs. When the…
Are You Rational About Sample Frequency and Process Behavior Charts?How you sample your process matters
Mon, 06/05/2023 - 12:03
The keys to effective process behavior charts are rational sampling and rational subgrouping. As implied by the word rational, we must use our knowledge of the context to collect and organize data in a way that answers the interesting questions.…
Do Process Behavior Charts Need Warning Limits?The consequences of trying to fine-tune your charts
Mon, 05/01/2023 - 12:03
Ever since 1935 people have been trying to fine-tune Walter Shewhart’s simple but sophisticated process behavior chart. One of these embellishments is the use of two-sigma “warning” limits. This column will consider the theoretical and practical…
The Distribution of Measurement ErrorThe origin of the error function
Mon, 04/03/2023 - 12:03
As the foundations of modern science were being laid, the need for a model for the uncertainty in a measurement became apparent. Here we look at the development of the theory of measurement error and discover its consequences. The problem may be…
Interpreting Data in ContextUsing process behavior charts in a clinical setting
Mon, 03/06/2023 - 12:03
In memory of Al Phadt, Ph.D. This article is a reprint of a paper Al and I presented several years ago. It illustrates how the interpretation and visual display of data in their context can facilitate discovery. Al’s integrated approach is a classic…
Kurtosis VisualizedNo one really understands kurtosis. Here’s why.
Mon, 02/06/2023 - 12:03
The shape parameters for a probability model are called skewness and kurtosis. While skewness at least sounds like something we might understand, kurtosis simply sounds like jargon. Here we’ll use some examples to visualize just what happens to a…
Skewness VisualizedWhat you think you know may not be so
Mon, 01/09/2023 - 12:03
The computation for skewness does not fully describe everything that happens as a distribution becomes more skewed. Here we shall use some examples to visualize just what skewness does—and does not—involve. The mean for a probability model…