Content by Donald J. Wheeler
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… What Is the Precision to Tolerance Ratio?And does it define a good measurement system?
Thu, 07/05/2012 - 11:13
In a class last month I was asked to explain a number that occurs in some measurement system evaluations and which is known as the precision to tolerance ratio (P/T ratio). As I will show in this column, it turns out to be related to the capability… Analysis Using Few Data, Part 1Some of these batches are not like the others…
Mon, 06/04/2012 - 13:06
Editor--Part 2 of this article can be found here.
In some industries a few test batches will be produced prior to going into production. When this happens, a critical question is: “Are all of the test batches alike?” With only one value per batch,… When Do We Use Subgrouped Data?A problem with service-sector average charts
Mon, 04/30/2012 - 11:32
When the data come along one value at a time, we tend to put them on a chart for individual values (an XmR chart). Since virtually all business and managerial data occur one value at a time, the primary chart for service-sector data is the XmR chart… When Should We Compute New Limits?How to use the limits to track the process
Mon, 04/02/2012 - 14:51
Last month in “Exact Answers to the Wrong Questions” we looked at how we can compute useful limits with as few as six to 10 values. In this column I would like to consider the question of how to use the limits on a process behavior chart to…
