dcsimg
 

Tips and Techniques for Statistics and Quality Improvement

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

Here's a shocking finding from the most recent ASQ Global State of Quality report: The higher you rise in your organization's leadership, the less often you receive reports about quality metrics. Only 2% of senior executives get daily quality reports, compared to 33% of front-line staff members.   A quarter of the senior executives reported getting quality metrics on a monthly basis, at least. But... Continue Reading
We like to host webinars, and our customers and prospects like to attend them. But when our webinar vendor moved from a pay-per-person pricing model to a pay-per-webinar pricing model, we wanted to find out how to maximize registrations and thereby minimize our costs. We collected webinar data on the following variables: Webinar topic Day of week Time of day – 11 a.m. or 2 p.m. Newsletter promotion –... Continue Reading
I’ve written about the importance of checking your residual plots when performing linear regression analysis. If you don’t satisfy the assumptions for an analysis, you might not be able to trust the results. One of the assumptions for regression analysis is that the residuals are normally distributed. Typically, you assess this assumption using the normal probability plot of the residuals. Are... Continue Reading
I got lost a lot as a child. I got lost at malls, at museums, Christmas markets, and everywhere else you could think of. Had it been in fashion to tether children to their parents at the time, I'm sure my mother would have. As an adult, I've gotten used to using a GPS device to keep me from getting lost. The Assistant in Minitab is like your GPS for statistics. The Assistant is there to provide you... Continue Reading
Halloween's right around the corner, so here's a scary thought for the statistically minded: That pattern in your time series plot? Maybe it's just a ghost. It might not really be there at all.   That's right. The trend that seems so evident might be a phantom. Or, if you don't believe in that sort of thing, chalk it up to the brain's desire to impose order on what we see, even when it doesn't... Continue Reading
A few weeks ago I looked at how the preseason college football poll influences the rankings at the end of the year. I found that for the most part, the teams that ranked higher in the preseason tend to be ranked higher going into the postseason. So if Team A and Team B both finish the regular season undefeated, the team that was ranked higher in the preseason tends to be the one ranked higher... Continue Reading
Suppose that you plan to source a substantial amount of parts or subcomponents from a new supplier. To ensure that their quality level is acceptable to you, you might want to assess the capability levels (Ppk and Cpk indices) of their manufacturing processes and check whether their critical process parameters are fully under control (using control charts). If you are not sure about the efficiency... Continue Reading
In quality initiatives such as Six Sigma, practitioners often need to assess the capability of a process to make sure it can meet specifications, or to verify that it produces good parts. While many Minitab users are familiar with the capability analysis tools in the Stat menu and in Minitab’s Assistant, the Assistant includes a less-frequently used feature—the Capability Snapshot. What Is the... Continue Reading
Astronomy is cool! And, it’s gotten even more exciting with the search for exoplanets. You’ve probably heard about newly discovered exoplanets that are extremely different from Earth. These include hot Jupiters, super-cold iceballs, super-heated hellholes, very-low-density puffballs, and ultra-speedy planets that orbit their star in just hours. And then there is PSR J1719-1438 which has the mass... Continue Reading
I’m always on the lookout for statistical news, so I was excited by the recent Matt Phillips article on Quartz titled Painfully, American families are learning the difference between median and mean. Phillips' allegory about Warren Buffet walking into a skid row bar makes a nice illustration of the statistical question about how outliers affect the mean. (If you want more on the mean and... 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
Keeping your vehicle fueled up is expensive. Maximizing the miles you get per gallon of fuel saves money and helps the environment, too.  But knowing if you're getting good mileage requires some data analysis, which gives us a good opportunity to apply one of the common tools used in Six Sigma -- the I-MR (individuals and moving range) control chart to daily life.    Finding Trends or Unusual... Continue Reading
Using statistical techniques to optimize manufacturing processes is quite common now, but using the same approach on social topics is still an innovative approach. For example, if our objective is to improve student academic performances, should we increase teachers wages or would it be better to reduce the number of students in a class? Many social topics (the effect of increasing the minimum... Continue Reading
The no-hitter is one of the most impressive feats in baseball. It’s no easy task to face more than 27 batters without letting one of them get a hit. So naturally, no-hitters don’t occur very often. In fact, since 1900 there has been an average of only about 2 no-hitters per year. But what if you had the opportunity to bet that one wouldn’t occur? That’s exactly what happened to sportswriter C. Trent... Continue Reading
You might not have known, but September is Attendance Awareness Month. Specifically, attendance of children at American public schools. The organization Attendance Works recently came out with a report that highlights the learning gap between students with strong attendance and students with poor attendance. Statistical software helps us quickly and easily create graphs that make it easier to... 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
In my previous post, I described how I was asked to weigh in on the ethics of researchers (DeStefano et al. 2004) who reportedly discarded data and potentially set scientific knowledge back a decade. I assessed the study in question and found that no data was discarded and that the researchers used good statistical practices. In this post, I assess a study by Brian S. Hooker that was... Continue Reading
The other day I received a request from a friend to look into a new study in a peer reviewed journal that found a link between MMR vaccinations and an increased risk of autism in African Americans boys. To draw this conclusion, the new study reanalyzed data that was discarded a decade ago by a previous study. My friend wanted to know, from a statistical perspective, was it unethical for the... Continue Reading
Did you just go shopping for school supplies? If you did, you’ve participated in what’s become the second biggest spending season of the year in the United States, according to the National Retail Federation (NRF). The trends and analysis are so interesting to the NRF that they actually add questions about back-to-school shopping to two monthly consumer surveys. The two surveys have different... Continue Reading
Previously, I showed why there is no R-squared for nonlinear regression. Anyone who uses nonlinear regression will also notice that there are no P values for the predictor variables. What’s going on? Just like there are good reasons not to calculate R-squared for nonlinear regression, there are also good reasons not to calculate P values for the coefficients. Why not—and what to use instead—are the... Continue Reading