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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.

After my husband’s most recent visit to the dentist, he returned home cavity-free...and with a $150 electric toothbrush in hand.  I wanted details. It began innocently. His dreaded trip to the dentist ended in high praise for no cavities and only a warning to floss more. That prompted my programming-and-automation-obsessed husband, still in the chair, to exclaim, "I wish there was a way to automate... Continue Reading
The easiest way to determine the probability of being born on a certain day is to assume that every day of the year has an equal probability of being a birthday. But academic scholarship tends to point to seasonal variation in births. If you average statistics from the United Nations, the seasonality in the United States of America from 1969 to 2013, excluding 1976 and 1977, looks like this: Seeing... Continue Reading

7 Deadly Statistical Sins Even the Experts Make

Do you know how to avoid them?

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You have a column of categorical data. Maybe it’s a column of reasons for production downtime, or customer survey responses, or all of the reasons airlines give for those riling flight delays. Whatever type of qualitative data you may have, suppose you want to find the most common categories. Here are three different ways to do that: 1. Pareto Charts Pareto Charts easily help you separate the vital... Continue Reading
I’ve written about R-squared before and I’ve concluded that it’s not as intuitive as it seems at first glance. It can be a misleading statistic because a high R-squared is not always good and a low R-squared is not always bad. I’ve even said that R-squared is overrated and that the standard error of the estimate (S) can be more useful. Even though I haven’t always been enthusiastic about... Continue Reading
Mind the gap. It's is an important concept to bear in mind whilst traveling on the Tube in London, the T in Boston, the Metro in Washington, D.C., etc. But how many of us remember to mind the gap when we create an interval plot in Minitab Statistical Software? Not too many of us, I'd wager. And it's a shame, too. When you travel on the subway, minding the gap means giving thoughtful consideration... Continue Reading
When running a binary logistic regression and many other analyses in Minitab, we estimate parameters for a specified model based on the sample data that has been collected. Most of the time, we use what is called Maximum Likelihood Estimation. However, based on specifics within your data, sometimes these estimation methods fail. What happens then? Specifically, during binary logistic regression, an... Continue Reading
What is an interaction? It’s when the effect of one factor depends on the level of another factor. Interactions are important when you’re performing ANOVA, DOE, or a regression analysis. Without them, your model may be missing an important term that helps explain variability in the response! For example, let’s consider 3-point shooting in the NBA. We previously saw that the number of 3-point... Continue Reading
In my last post, I looked at viewership data for the five seasons of HBO’s hit series Game of Thrones. I created a time series plot in Minitab that showed how viewership rose season by season, and how it varied episode by episode within each season.   My next step is to fit a statistical model to the data, which I hope will allow me to predict the viewing numbers for future episodes.    I am going to... Continue Reading
If you’ve not heard of the TV series Game of Thrones, you must have been living on Mars for the past few years! An adaptation of the fantasy series A Song of Ice and Fire by George R. R. Martin, the show is an epic tale of the political conflicts and wars between noble houses in the fictional continents of Westeros and Essos over who sits in the Iron Throne, and thus rules the whole realm. (It was... Continue Reading
For the majority of my career with Minitab, I've had the opportunity to speak at conferences and other events somewhat regularly. I thought some of my talks were pretty good, and some were not so good (based on ratings, my audiences didn't always agree with either—but that's a topic for another post). But I would guess that well over 90% of the time, my proposals were accepted to be presented at... Continue Reading
While the roots of Lean Six Sigma and other quality improvement methodologies are in manufacturing, it’s interesting to see how other organizational functions and industries apply LSS tools successfully. Quality improvement certainly has moved far beyond the walls of manufacturing plants! For example, I recently had the opportunity to talk to Drew Mohler, a Lean Six Sigma black belt and senior... Continue Reading
There's nothing like a boxplot, aka box-and-whisker diagram, to get a quick snapshot of the distribution of your data. With a single glance, you can readily intuit its general shape, central tendency, and variability. To easily compare the distribution of data between groups, display boxplots for the groups side by side. Visually compare the central value and spread of the distribution for each... Continue Reading
In statistics, there are things you need to do so you can trust your results. For example, you should check the sample size, the assumptions of the analysis, and so on. In regression analysis, I always urge people to check their residual plots. In this blog post, I present one more thing you should do so you can trust your regression results in certain circumstances—standardize the continuous... Continue Reading
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 five years, and the fact that this analysis targeted and harmed just one person makes it more... 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
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