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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.

If you want to convince someone that at least a basic understanding of statistics is an essential life skill, bring up the case of Lucia de Berk. Hers is a story that's too awful to be true—except that it is completely true. A flawed analysis irrevocably altered de Berk's life and kept her behind bars for a full decade, and the fact that this analysis targeted and harmed just one person makes it... Continue Reading
When I wrote How to Calculate B10 Life with Statistical Software, I promised a follow-up blog post that would describe how to compute any “BX” lifetime. In this post I’ll follow through on that promise, and in a third blog post in this series, I will explain why BX life is one of the best measures you can use in your reliability analysis. As a refresher, B10 life refers to the time at which 10% of... Continue Reading

7 Deadly Statistical Sins Even the Experts Make

Do you know how to avoid them?

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In this post, I’ll address some common questions we’ve received in technical support about the difference between fitted and data means, where to find each option within Minitab, and how Minitab calculates each. First, let’s look at some definitions. It’s useful to have an example, so I’ll be using the Light Output data set from Minitab’s Data Set Library, which includes a description of the sample... Continue Reading
If you follow the news in the United States then you’ve heard that there’s a water crisis in Flint, Michigan. Although there’s going to continue to be debate about how much ethics played a role in the data collection practices, it’s worthwhile to at least be ready to perform the correct analysis on the data when you have it. Here’s how you can use Minitab to be like a citizen data scientist... Continue Reading
If you need to assess process performance relative to some specification limit(s), then process capability is the tool to use. You collect some accurate data from a stable process, enter those measurements in Minitab, and then choose Stat > Quality Tools > Capability Analysis/Sixpack or Assistant > Capability Analysis. Now, what about sorting the data? I’ve been asked “why does Cpk change when I... Continue Reading
In the world of linear models, a hierarchical model contains all lower-order terms that comprise the higher-order terms that also appear in the model. For example, a model that includes the interaction term A*B*C is hierarchical if it includes these terms: A, B, C, A*B, A*C, and B*C. Fitting the correct regression model can be as much of an art as it is a science. Consequently, there's not always a... Continue Reading
In my time at Minitab, I’ve gotten a good understanding of what types of graphs users create. Everyone knows about histograms, bar charts, and time series plots. Even relatively less familiar plots like the interval plot and individual value plot are still used quite often. However, one of the most underutilized graphs we have available is the area graph. If you’re not familiar with an Area... Continue Reading
Any time you see a process changing, it's important to determine why. Is it indicative of a long term trend, or is it a fad that you can ignore since it will be gone shortly?  For example, in the 2014 NBA Finals, the San Antonio Spurs beat the two-time defending champion Miami Heat by attempting more 3-pointers (23.6 per game) than any championship team in league history. In the 2015 regular... Continue Reading
Unless you live under a black country rock, you’ve no doubt heard that the world recently lost one of the greatest artists of our time, David Bowie. My memories of the Thin White Duke go all the way back to my formative years. I recall his music echoing through the halls of our house as I crooned along whilst doing the chores. Then as now, Bowie’s creativity and energy inspired me and helped me do... Continue Reading
Ahh, bottled water. Refreshing, convenient...and sometimes pricey. Or in my case, I should say usually pricey. Confession: I’m a sucker for water that comes in the “pretty” plastic bottles, and my experience is that the pretty-bottle brands are usually the pricier ones. Does bottled water cost increase with the fanciness of the bottle? Well, that could be an analysis for a different day … My... Continue Reading
How deeply has statistical content from Minitab blog posts (or other sources) seeped into your brain tissue? Rather than submit a biopsy specimen from your temporal lobe for analysis, take this short quiz to find out. Each question may have more than one correct answer. Good luck! Which of the following are famous figure skating pairs, and which are methods for testing whether your data follow a... Continue Reading
If you perform linear regression analysis, you might need to compare different regression lines to see if their constants and slope coefficients are different. Imagine there is an established relationship between X and Y. Now, suppose you want to determine whether that relationship has changed. Perhaps there is a new context, process, or some other qualitative change, and you want to determine... Continue Reading
At the start of a new year, I like to look for data that’s labeled 2016. While it’s not necessarily new for 2016, one of the first data sets I found was healthcare.gov’s data about qualified health and stand-alone dental plans offered through their site. Now, there’s lots of fun stuff to poke around in a data set this size—there are over 90,000 records on more than 140 variables. But to start out I... Continue Reading
In an earlier post, I shared an overview of acceptance sampling, a method that lets you evaluate a sample of items from a larger batch of products (for instance, electronics components you've sourced from a new supplier) and use that sample to decide whether or not you should accept or reject the entire shipment.  There are two approaches to acceptance sampling. If you do it by attributes, you... Continue Reading
Now that we've seen how easy it is to create plans for acceptance sampling by variables, and to compare different sampling plans, it's time to see how to actually analyze the data you collect when you follow the sampling plan.  If you'd like to follow along and you're not already using Minitab, please download the free 30-day trial.  Collecting the Data for Acceptance Sampling by Variable If you'll... Continue Reading
In my last post, I showed how to use Minitab Statistical Software to create an acceptance sampling plan by variables, using the scenario of a an electronics company that receives monthly shipments of LEDs that must have soldering leads that are at least 2 cm long. This time, we'll compare that plan with some other possible options.  The variables sampling plan we came up with to verify the... Continue Reading
Earlier, I shared an overview of acceptance sampling. Now we'll look at how to do acceptance sampling by variables, facilitated by the tools in Minitab Statistical Software. If you're not already using it and you'd like to follow along, you can get the free 30-day trial version.  In contrast to acceptance sampling by attributes, where inspectors make judgment calls about defective items,... Continue Reading
If you're just getting started in the world of quality improvement, or if you find yourself in a position where you suddenly need to evaluate the quality of incoming or outgoing products from your company, you may have encountered the term "acceptance sampling." It's a statistical method for evaluating the quality of a large batch of materials from a small sample of items, which statistical softwar... Continue Reading
In my last post, I walked through the steps to install Minitab 17 on a Mac using Apple Boot Camp.  Minitab 17 can also be installed on a Mac using desktop virtualization software. In addition to your Mac, you’ll need: A copy of Windows 7 or later version ISO Minitab 17 Statistical Software Desktop virtualization software allows you to install and use Windows on your Intel-based Mac without requiring... Continue Reading
While Minitab 17 is currently a Windows-only application, there are people who only have a Mac available for the installation who also find they need to use Minitab 17.  It is possible to run Minitab 17 on a Macintosh, though the steps involved in the installation can seem a little daunting at first. In the Technical Support department, we sometimes hear reluctance in people’s voices when we throw... Continue Reading