Patrick Runkel

Communicating statistical concepts quickly and clearly is just as critical as being able to calculate them, so I focus on quick, practical insights about statistics to help you make data-driven decisions. Continue Reading »

If you use ordinary linear regression with a response of count data, if may work out fine (Part 1), or you may run into some problems (Part 2). Given that a count response could be problematic, why not use a regression procedure developed to handle a response of counts? A Poisson regression analysis is designed to analyze a regression model with a count response. First, let's try using Poisson... Continue Reading
My previous post showed an example of using ordinary linear regression to model a count response. For that particular count data, shown by the blue circles on the dot plot below, the model assumptions for linear regression were adequately satisfied. But frequently, count data may contain many values equal or close to 0. Also, the distribution of the counts may be right-skewed. In the quality field,... Continue Reading
Ever use dental floss to cut soft cheese? Or Alka Seltzer to clean your toilet bowl? You can find a host of nonconventional uses for ordinary objects online. Some are more peculiar than others. Ever use ordinary linear regression to evaluate a response (outcome) variable of counts?  Technically, ordinary linear regression was designed to evaluate a a continuous response variable. A continuous... Continue Reading
I've never understood the fascination with selfies. Maybe it's because I'm over 50. After surviving the slings and arrows of a half a century on Earth, the minute or two I spend in front of the bathroom mirror each morning is more than enough selfie time for me. Still, when I heard that Microsoft had an online app that estimates the age of any face on a photo, I was intrigued. How would the app... Continue Reading
It’s usually not a good idea to rely solely on a single statistic to draw conclusions about your process. Do that, and you could fall into the clutches of the “duck-rabbit” illusion shown here: If you fix your eyes solely on the duck, you’ll miss the rabbit—and vice-versa. If you're using Minitab Statistical Software for capability analysis, the capability indices Cp and Cpk are good examples of... Continue Reading
I always knew I was different. Even as a kid. “Is that me? Way out there in left field?” I asked the doc. “Yes,” he nodded, as he looked at my chart. “I used brushing to identify you on the graph.” I wasn’t sure I liked getting brushed. It felt like my true identify was being detected and displayed in a window for all to see. The doctor must have sensed my discomfort. “It’s not uncommon—even for those... Continue Reading
Right now I’m enjoying my daily dose of morning joe. As the steam rises off the cup, the dark rich liquid triggers a powerful enzyme cascade that jump-starts my brain and central nervous system, delivering potent glints of perspicacity into the dark crevices of my still-dormant consciousness. Feels good, yeah! But is it good for me? Let’s see what the studies say… Drinking more than 4 cups of coffee... Continue Reading
If you’re not a statistician, looking through statistical output can sometimes make you feel a bit like Alice in Wonderland. Suddenly, you step into a fantastical world where strange and mysterious phantasms appear out of nowhere.   For example, consider the T and P in your t-test results. “Curiouser and curiouser!” you might exclaim, like Alice, as you gaze at your output. What are these values,... Continue Reading
"He looks just like his father...and mother!" Popular morphing sites online let you visualize the hypothetical offspring of some very unlikely couples. The baby of Albert Einstein and Kim Kardashian (Kimbert?) would presumably look something like the image shown at right. What happens if you morph the features of two different graphs? For example, what would the baby of a time series plot and... Continue Reading
The word kurtosis sounds like a painful, festering disease of the gums. But the term actually describes the shape of a data distribution. Frequently, you'll see kurtosis defined as how sharply "peaked" the data are. The three main types of kurtosis are shown below. Lepto means "thin" or "slender" in Greek. In leptokurtosis, the kurtosis value is high. Platy means "broad" or "flat"—as in duck-billed pl... Continue Reading
Do you suffer from PAAA (Post-Analysis Assumption Angst)? You’re not alone. Checking the required assumptions for a statistical  analysis is critical. But if you don’t have a Ph.D. in statistics, it can feel more complicated and confusing than the primary analysis itself. How does the cuckoo egg data, a common sample data set often used to teach analysis of variance, satisfy the following formal... Continue Reading
If you teach statistics or quality statistics, you’re probably already familiar with the cuckoo egg data set. The common cuckoo has decided that raising baby chicks is a stressful, thankless job. It has better things to do than fill the screeching, gaping maws of cuckoo chicks, day in and day out. So the mother cuckoo lays her eggs in the nests of other bird species. If the cuckoo egg is similar... Continue Reading
You know what really gets on my nerves? A lot of things. That slow, slinky way that cats walk by. Grrrr. The rude, abrupt arrival of delivery persons in their obnoxiously loud trucks. (Why do they always pull up just as I’m settling down for a nap?) Grrrr. Total strangers who reach down and poke me with fat, clumsy fingers that reek of antibacterial soap. Grrrr. And this one always gets my dander up:... Continue Reading
These days, my memory isn't what it used to be. Besides that, my memory isn't what it used to be.  But my incurable case of CRS (Can't Remember Stuff) is not nearly as bad as that of the exponential distribution. When modelling failure data for reliability analysis, the exponential distribution is completely memoryless. It retains no record of the previous failure of an item. That might sound like a... Continue Reading
A famous classical Chinese poem from the Song dynasty describes the views of a mist-covered mountain called Lushan. The poem was inscribed on the wall of a Buddhist monastery by Su Shi, a renowned poet, artist, and calligrapher of the 11th century. Deceptively simple, the poem captures the illusory nature of human perception.      Written on the Wall of West Forest Temple                               ... Continue Reading
My previous post examined how an equivalence test can shift the burden of proof when you perform hypothesis test of the means. This allows you to more rigorously test whether the process mean is equivalent to a target or to another mean. Here’s another key difference: To perform the analysis, an equivalence test requires that you first define, upfront, the size of a practically important difference... Continue Reading
With more options, come more decisions. With equivalence testing added to Minitab 17, you now have more statistical tools to test a sample mean against target value or another sample mean. Equivalence testing is extensively used in the biomedical field. Pharmaceutical manufacturers often need to test whether the biological activity of a generic drug is equivalent to that of a brand name drug that... Continue Reading
B'gosh n' begorrah, it's St. Patrick's Day today! The day that we Americans lay claim to our Irish heritage by doing all sorts of things that Irish people never do. Like dye your hair green. Or tell everyone what percentage Irish you are. Despite my given name, I'm only about 15% Irish. So my Irish portion weighs about 25 pounds. It could be the portion that hangs over my belt due to excess potatoes... Continue Reading
By popular demand, Release 17 of Minitab Statistical Software comes with a new graphical analysis called the Bubble Plot. This exploratory tool is great for visualizing the relationships among three variables on a single plot. To see how it works, consider the total medal count by country from the recently completed 2014 Olympic Winter Games. Suppose I want to explore whether there might be a... Continue Reading
If you regularly perform regression analysis, you know that R2 is a statistic used to evaluate the fit of your model. You may even know the standard definition of R2: the percentage of variation in the response that is explained by the model. Fair enough. With Minitab Statistical Software doing all the heavy lifting to calculate your R2 values, that may be all you ever need to know. But if you’re... Continue Reading