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Tips and Techniques for Statistics and Quality Improvement

Blog posts and articles about using Minitab software in quality improvement projects, research, and more.

"Data! Data! Data! I can't make bricks without clay."  — Sherlock Holmes, in Arthur Conan Doyle's The Adventure of the Copper Beeches Whether you're the world's greatest detective trying to crack a case or a person trying to solve a problem at work, you're going to need information. Facts. Data, as Sherlock Holmes says.  But not all data is created equal, especially if you plan to analyze as part of... Continue Reading
At the end of my previous post, aspiring statisticians Woodrow "Woody" Stem and August "Russell" Leaf, creators of the famed Stem-and-Leaf plot, were in bad shape. They had beaten each other statsless after an argument about the challenge given to them by their mentor, Dr. Histeaux Graham. That challenge: to devise a simple, yet elegant way to examine the distribution of values in a sample. After... Continue Reading
Over the last year or so I’ve heard a lot of people asking, “How can I calculate B10 life in Minitab?” Despite being a statistician and industrial engineer (mind you, one who has never been in the field like the customers asking this question) and having taken a reliability engineering course, I’d never heard of B10 life. So I did some research. The B10 life metric originated in the ball and roller... Continue Reading
Throughout the college football season, I’ve been looking at the influence of the preseason AP Poll on rankings later in the season. Each analysis found a positive association between preseason rankings and the current rankings. That is, between top-ranked teams with a similar number of losses, teams ranked higher in the preseason are also ranked higher in current polls. The biggest exception is... Continue Reading
Stepwise regression and best subsets regression are both automatic tools that help you identify useful predictors during the exploratory stages of model building for linear regression. These two procedures use different methods and present you with different output. An obvious question arises. Does one procedure pick the true model more often than the other? I’ll tackle that question in this post. Fi... Continue Reading
Sure, Minitab Statistical Software is powerful and easy to use, but did you know that it’s also magic? One of the illusions that Minitab can peform is the world famous disappearing-reappearing-analysis-settings act. Of course, as with many illusions, it’s not so hard once you know the trick. In this case, it’s downright easy once you know about Minitab project files. If you’ve done any work in... Continue Reading
Minitab's capability analysis output gives you estimates of the capability indices Ppk and Cpk, and we receive many questions about the difference between them. Some of my colleagues have taken other approaches to explain the difference between Ppk and Cpk, so I wanted to show you how they differ by detailing precisely how each one is calculated.  When you're using statistical softwarelike Minitab,... Continue Reading
In technical support, we frequently receive calls from Minitab users who have questions about the differences between Cpk and Ppk.  Michelle Paret already wrote a great post about the differences between Cpk and Ppk, but it also helps to have a better understanding of the math behind these numbers. So in this post I will show you how to calculate Ppk using Minitab’s default settings when the... Continue Reading
Have you ever had a probability plot that looks like this? The probability plot above is based on patient weight (in pounds) after surgery minus patient weight (again, in pounds) before surgery. The red line appears to go through the data, indicating a good fit to the Normal, but there are clusters of plotting points at the same measured value. This occurs on a probability plot when there are many... Continue Reading
Greetings fair reader. In the past, I've written several posts with practical tips related to Minitab graphs, such as: How to discuss the sensitive issue of P charts and Laney P' charts with your doctor How to use a G chart to monitor parenting success How to use a scatterplot to start your own doomsday cult In this post, I thought I'd take a step back and explore the historical side of Minitab's... Continue Reading
Hi everyone! Over the past month, I fielded some interesting customer calls regarding control chart creation and editing. I wanted to share these potential scenarios with you in hopes that you will find them informative and useful. For these scenarios, I used the XBar-R chart as my template, but you could easily apply them to many of the other control charts in Minitab.  Scenario 1: Create a... Continue Reading
My siblings occasionally remind me that because I’m getting older, one day, my metabolism is going to collapse. When that day comes, consuming mass quantities of food will surely lead to the collapse of my body, mind, and soul. But, as that day is coming slowly, on Thanksgiving, I’m an every-pie-kind-of-guy. Now, I know what you’re thinking. It’s Thanksgiving. I’ve just mentioned pies. We’re going... Continue Reading
Last week, thanks to the collective effort from many people, we held very successful events in Guadalajara and Mexico City, which gave us a unique opportunity to meet with over 300 Spanish-speaking Minitab users. They represented many different industries, including automotive, textile, pharmaceutical, medical devices, oil and gas, electronics, and mining, as well as academic institutions and... Continue Reading
Everyone who analyzes data regularly has the experience of getting a worksheet that just isn't ready to use. Previously I wrote about tools you can use to clean up and elminate clutter in your data and reorganize your data.  In this post, I'm going to highlight tools that help you get the most out of messy data by altering its characteristics. Know Your Options Many problems with data don't become... Continue Reading
College basketball season tips off today, and for the second straight season Kentucky is the #1 ranked preseason team in the AP poll. Last year Kentucky did not live up to that ranking in the regular season, going 24-10 and earning a lowly 8 seed in the NCAA tournament. But then, in the tournament, they overachieved and made a run all the way to the championship game...before losing... Continue Reading
Analysis of variance (ANOVA) is great when you want to compare the differences between group means. For example, you can use ANOVA to assess how three different alloys are related to the mean strength of a product. However, most ANOVA tests assess one response variable at a time, which can be a big problem in certain situations. Fortunately, Minitab statistical software offers a... Continue Reading
Control charts are excellent tools for looking at data points that seem unusual and for deciding whether they're worthy of investigation. If you use control charts frequently, then you're used to the idea that if certain subgroups reflect temporary abnormalities, you can leave them out when you calculate your center line and control limits. If you include points that you already know are... Continue Reading
In my last post, I wrote about making a cluttered data set easier to work with by removing unneeded columns entirely, and by displaying just those columns you want to work with now. But too much unneeded data isn't always the problem. What can you do when someone gives you data that isn't organized the way you need it to be?   That happens for a variety of reasons, but most often it's because the... Continue Reading
Isn't it great when you get a set of data and it's perfectly organized and ready for you to analyze? I love it when the people who collect the data take special care to make sure to format it consistently, arrange it correctly, and eliminate the junk, clutter, and useless information I don't need.   You've never received a data set in such perfect condition, you say? Yeah, me neither. But I can... Continue Reading
A few weeks ago my colleague Cody Steele illustrated how the same set of data can appear to support two contradictory positions. He showed how changing the scale of a graph that displays mean and median household income over time drastically alters the way it can be interpreted, even though there's no change in the data being presented. When we analyze data, we need to present the results in... Continue Reading