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…
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…
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…
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…
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…
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…
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…
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…
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.…
Part 2 How to avoid asking the wrong question
Mon, 08/04/2014 - 09:11
One of the common tools of quality assurance is acceptance sampling. Acceptance sampling uses the observed properties of a sample drawn from a lot or batch to make a decision about whether to accept or reject that lot or batch. While the textbooks…