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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.

In statistics, t-tests are a type of hypothesis test that allows you to compare means. They are called t-tests because each t-test boils your sample data down to one number, the t-value. If you understand how t-tests calculate t-values, you’re well on your way to understanding how these tests work. In this series of posts, I'm focusing on concepts rather than equations to show how t-tests work.... Continue Reading
In the first part of this series, we looked at a case study where staff at a hospital used ATP swab tests to test 8 surfaces for bacteria in 10 different hospital rooms across 5 departments. ATP measurements below 400 units pass the swab test, while measurements greater than or equal to 400 units fail the swab test and require further investigation. I offered two tips on exploring and visualizing... Continue Reading

7 Deadly Statistical Sins Even the Experts Make

Do you know how to avoid them?

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Working with healthcare-related data often feels different than working with manufacturing data. After all, the common thread among healthcare quality improvement professionals is the motivation to preserve and improve the lives of patients. Whether collecting data on the number of patient falls, patient length-of-stay, bed unavailability, wait times, hospital acquired-infections, or readmissions,... Continue Reading
We often receive questions about moving ranges because they're used in various tools in our statistical software, including control charts and capability analysis when data is not collected in subgroups. In this post, I'll explain what a moving range is, and how a moving range and average moving range are calculated. A moving range measures how variation changes over time when data are collected as... Continue Reading
Along with the explosion of interest in visualizing data over the past few years has been an excessive focus on how attractive the graph is at the expense of how useful it is. Don't get me wrong...I believe that a colorful, modern graph comes across better than a black-and-white, pixelated one. Unfortunately, however, all the talk seems to be about the attractiveness and not the value of the... Continue Reading
As a recent graduate from Arizona State University with a degree in Business Statistics, I had the opportunity to work with students from different areas of study and help analyze data from various projects for them. One particular group asked for help analyzing online survey data they had gathered from other students, and they wanted to see if their new student program was beneficial. I would... Continue Reading
Did you know about the Minitab Network group on LinkedIn? It’s the one managed by Eston Martz, who also edits the Minitab blog. I like to see what the members are talking about, which recently got me into some discussions about Raman spectroscopy data. Not having much experience with Raman spectroscopy data, I thought I’d learn more about it and found the RRUFFTM Project. The idea is that if you... Continue Reading
Getting your data from Excel into Minitab Statistical Software for analysis is easy, especially if you keep the following tips in mind. Copy and Paste To paste into Minitab, you can either right-click in the worksheet and choose Paste Cells or you can use Control-V. Minitab allows for 1 row of column headers, so if you have a single row of column info (or no column header info), then you can quickly... Continue Reading
T-tests are handy hypothesis tests in statistics when you want to compare means. You can compare a sample mean to a hypothesized or target value using a one-sample t-test. You can compare the means of two groups with a two-sample t-test. If you have two groups with paired observations (e.g., before and after measurements), use the paired t-test. How do t-tests work? How do t-values fit in? In this... Continue Reading
Depending on how often and when you use statistical software like Minitab, there may be specific tools or a group of tools you find yourself using over and over again. You may have to do a monthly report, for instance, for which you use one tool in our Basic Statistics menu, another in Quality Tools, and a third in Regression.  But there are a lot of functions and capabilities in our software, and... Continue Reading
When it comes to statistical analyses, collecting a large enough sample size is essential to obtaining quality results. If your sample size is too small, confidence intervals may be too wide to be useful, linear models may lack necessary precision, and control charts may get so out of control that they become self-aware and rise up against humankind. Okay,that last point may have been... Continue Reading
People say that I overthink everything. I've given this assertion considerable thought, and I don't believe that it is true. After all, how can any one person possibly overthink every possible thing in just one lifetime? For example, suppose I live 85 years. That's 2,680,560,000 seconds (85 years x 365 days per year x 24 hours per day x 60 min per hour x 60 seconds per minute). I'm asleep about a... Continue Reading
Did you know that Minitab provides several tools you can use to view patterns in data over time? If you want to examine, say, monthly sales for your company, or even how the number of patients admitted to your hospital changes throughout the year, then these tools are for you! 1. Time Series Plot Time series plots are often used to examine daily, weekly, seasonal or annual variations, or... Continue Reading
About a year ago, a reader asked if I could try to explain degrees of freedom in statistics. Since then,  I’ve been circling around that request very cautiously, like it’s some kind of wild beast that I’m not sure I can safely wrestle to the ground. Degrees of freedom aren’t easy to explain. They come up in many different contexts in statistics—some advanced and complicated. In mathematics, they're... Continue Reading
Five-point Likert scales are commonly associated with surveys and are used in a wide variety of settings. You’ve run into the Likert scale if you’ve ever been asked whether you strongly agree, agree, neither agree or disagree, disagree, or strongly disagree about something. The worksheet to the right shows what five-point Likert data look like when you have two groups. Because Likert item data are... Continue Reading
Since opening a new office in Phoenix to support our customers on the West Coast, some evenings in Minitab technical support feel busier than others. (By evenings, I mean after 5:30 p.m. Eastern time, when the members of our tech support team in Pennsylvania go home for the day, and I become an office of one.) The variability in terms of days that felt extremely busy versus days that didn’t... Continue Reading
Allow me to make a confession up front: I won't hesitate to beat my kids at a game. My kids are young enough that in pretty much any game that is predominantly determined by skill and not luck, I can beat them—and beat them easily. This isn't some macho thing where it makes me feel good, and I suppose is only partially based in wanting them to handle both winning and losing well. It's just how I... Continue Reading
Most of us have heard a backwards way of completing a task, or doing something in the conventionally wrong order, described as “putting the cart before the horse.” That’s because a horse pulling a cart is much more efficient than a horse pushing a cart. This saying may be especially true in the world of statistics. Focusing on a statistical tool or analysis before checking out the condition of your... Continue Reading
In my last post, I discussed how a DOE was chosen to optimize a chemical-mechanical polishing process in the microelectronics industry. This important process improved the plant's final manufacturing yields. We selected an experimental design that let us study the effects of six process parameters in 16 runs. Analyzing the Design Now we'll examine the analysis of the DOE results after the actual... Continue Reading
I used to work in the manufacturing industry. Some processes were so complex that even a very experienced and competent engineer would not necessarily know how to identify the best settings for the manufacturing equipment. You could make a guess using a general idea of what should be done regarding the optimal settings, but that was not sufficient. You need very precise indications of the correct... Continue Reading