Donald Wheeler

Mar 5, 2024 | qualitydigest.com | Donald Wheeler

Over the past two months we’ve considered the properties of lognormal and gamma probability models. Both of these families contain the normal distribution as a limit. To complete our survey of widely used probability models, this column will look at Weibull distributions, a family that doesn’t contain the normal.

Feb 6, 2024 | qualitydigest.com | Donald Wheeler

Clear thinking and simplicity of analysis require concise, clear, and correct notions about probability models and how to use them. Here, we’ll examine the basic properties of the family of gamma and chi-square distributions that play major roles in the development of statistical techniques. An appreciation of how these probability models function will allow you to analyze your data with confidence that your results are reliable, solid, and correct.

Jan 4, 2024 | qualitydigest.com | Donald Wheeler

When do we need to fit a lognormal distribution to our skewed histograms? This article considers the basic properties of the lognormal family of distributions and reveals some interesting and time-saving characteristics that are useful when analyzing data. While software facilitates the use of lognormal distributions, the following formulas are given here in the interest of clarity of notation.

Nov 30, 2023 | qualitydigest.com | Donald Wheeler

Fourteen years ago I published “Do You Have Leptokurtophobia?” Based on the reaction to that column, the message was needed. In this column I would like to explain the symptoms of leptokurtophobia and the cure for this pandemic affliction. Leptokurtosis is a Greek word that literally means “thin mound.” It was used to describe those probability models that, near the mean, have a more rounded (or peaked) probability density function than that of a normal distribution.

Oct 26, 2023 | qualitydigest.com | Donald Wheeler

In last month’s column, we looked at how process-hyphen-control algorithms work with a process that is subject to occasional upsets. This column will consider how they work with a well-behaved process. Last month we saw that process adjustments can reduce variation when they are reacting to real changes in the process. To be clear, I know that process control technology is everywhere, and it allows us to do things we couldn’t otherwise do.