Content by Donald J. Wheeler
Contra Two SigmaThe consequences of using the wrong limits
Wed, 05/01/2013 - 10:27
You may occasionally encounter charts with two-sigma limits. The origins of this practice are not clear, and no real justification of this practice has been given in the literature. In this article, I will consider the theoretical and practical… Consistency ChartsSPC for measurement systems
Wed, 03/27/2013 - 11:15
What happens when we measure the same thing and get different values? How can we ever use such a measurement system to measure different things? By thinking of measurements as the product and the measurement procedure as the process we can use the… Right and Wrong Ways to Use SpecificationsShould you sort it or adjust it?
Wed, 02/27/2013 - 11:01
Last month I looked at "The Secret of Process Adjustment." This column will review the history and purpose of specifications and look at two common ways that specifications are used in practice. Using simple examples I will illustrate the right… The Secret of Process AdjustmentUnneeded adjustments always make things worse
Tue, 01/29/2013 - 13:30
When your process outcomes are not what you expect them to be it is common to adjust the process. This is not always appropriate. To understand when adjustments are appropriate, and when they are inappropriate, we will need to learn how to… But the Limits Are Too Wide!When the <em>XmR</em> Chart Doesn’t Seem to Work
Wed, 01/02/2013 - 13:13
Last month I described what makes the XmR chart work. This month I will describe some common failure modes for the XmR chart and show how they come from a failure to follow the two fundamental principles behind the XmR chart.
When administrative… What Makes the XmR Chart Work?How does it separate the signals from the noise?
Thu, 11/29/2012 - 13:39
There are two basic ideas or principles that need to be respected when creating a chart for individual values and a moving range (an XmR chart). This column will explain and illustrate these two principles for effective XmR charts.
The first… Analysis Using Few Data, Part 2Comparing the <em>W</em>-ratio test with a homogeneity chart
Wed, 10/24/2012 - 11:40
While an XmR chart is commonly used as a process behavior chart, it may also be used as a test of homogeneity for a finite number of values. This column explains the difference in these two uses of the XmR chart and compares the homogeneity chart… The Heavy-Tailed NormalAnswers to questions
Mon, 10/01/2012 - 13:38
I n my September column, I showed how the normal distribution is the distribution of maximum uncertainty. Now I will expand on that theme and answer the questions generated by that column.
Last month I demonstrated that the middle 91 percent of the… What They Forgot to Tell You About the Normal Distribution How the normal distribution has maximum uncertainty
Tue, 09/04/2012 - 16:03
There are two key aspects of the normal distribution that make it the central probability model in statistics. However, students seldom hear about these important aspects, and as a result they end up making many unnecessary mistakes. Read on to… What Is Leptokurtophobia?And why does it matter?
Mon, 07/30/2012 - 13:17
Three years ago this month Quality Digest Daily published my column, “Do You Have Leptokurtophobia?” Based on the reaction to that column, it contained a message that was needed. In this column I would like to explain the symptoms of…
