Content by Donald J. Wheeler
Rational SubgroupingThe conceptual foundation of process behavior charts
Mon, 06/01/2015 - 01:00
While the computations for a process behavior chart are completely general and very robust, the secret to using a process behavior chart effectively lies in the art of rational sampling and rational subgrouping. Rational subgrouping has to do with… The Parts-Per-Million ProblemWhen does a computation result in a number?
Mon, 05/11/2015 - 10:56
Parts per million (ppm) is part of the language of Six Sigma. It pervades the sales pitch and is used in all sorts of computations as a measure of quality. Yet what are the rules of arithmetic and statistics that govern the computation and usage of… Numerical JabberwockyNumbers that gyre and gymble in the wabe
Wed, 04/01/2015 - 16:40
The first axiom of data analysis is: “No data have meaning apart from their context.” Yet we all encounter measures that have been dreamed up without regard for either context or use. This column gives a couple of the more egregious examples I have… Probability LimitsA long standing controversy
Mon, 03/02/2015 - 13:04
Author clarification--3/5/2015:It appears that I somewhat overstated my case in this article. I had forgotten that there are some families of distributions where we can estimate the shape of a probability model using the statistics for location and/… Process Behavior Charts for Non-Normal Data, Part 2What happens to the range chart?
Tue, 02/03/2015 - 15:41
Whenever the original data pile up against a barrier or a boundary value, the histogram tends to be skewed and non-normal in shape. Last month in part one we found that this doesn’t appreciably affect the performance of process behavior charts for… Process Behavior Charts for Non-Normal Data, Part 1A guide for charts for location
Tue, 01/06/2015 - 15:24
Whenever the original data pile up against a barrier or a boundary value, the histogram tends to be skewed and non-normal in shape. In 1967 Irving W. Burr computed the appropriate bias correction factors for non-normal probability models. These… A Tale of Two ComparisonsAre these two processes the same?
Tue, 12/02/2014 - 11:58
Comparisons are often required as part of doing business. Are these two machines the same? Is this process the same as that process? Are two operators performing in the same way? In this article we will look at two ways of making these comparisons… Obey GravityIt’s the law
Mon, 11/03/2014 - 09:48
There is no virtue in obedience when we do not have a choice. But when we have a choice it helps to understand both the law and the reason behind the law. This column is about bad choices that are being made on a daily basis by the users of… Don’t We Need to Remove the Outliers?Characterization and estimation are different
Mon, 10/06/2014 - 10:56
Much of modern statistics is concerned with creating models which contain parameters that need to be estimated. In many cases these estimates can be severely affected by unusual or extreme values in the data. For this reason students are often… A Problem With Outlier TestsWhen can you really use Dixon’s test?
Tue, 09/02/2014 - 18:26
Outlier tests such as the W-ratio test and Dixon’s outlier test suffer from a problem that can mislead the user. This article will outline the problem and provide guidelines for the appropriate use of these tests.
Dixon’s outlier test
In 1953, W.…
