Blog posts and articles about statistical principles in quality improvement methods like Lean and Six Sigma.

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

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

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

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

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

Leading and trailing spaces in a data set are like termites in
your house. If you don’t realize they are there and you don’t get
rid of them, they’re going to wreak havoc.
Here
are a few easy ways to remove these pesky characters with Minitab Statistical
Software prior to analysis.
Data Import
If you’re importing data from Excel, a text file, or some other
file type:
Choose File > Open and select
your... Continue Reading

P values have been around for nearly a century and they’ve been
the subject of criticism since their origins. In recent years, the
debate over P values has risen to a fever pitch. In particular,
there are serious fears that P values are misused to such an extent
that it has actually damaged science.
In March 2016, spurred on by the growing concerns, the American
Statistical Association (ASA) did... Continue Reading

When you analyze a Gage R&R study in statistical
software, your results can be overwhelming. There are a lot of
statistics listed in Minitab's Session Window—what do they all
mean, and are they telling you the same thing?
If you don't know where to start, it can be hard to figure out
what the analysis is telling you, especially if your measurement
system is giving you some numbers you'd think are... Continue Reading