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.
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.
flawed analysis irrevocably altered de Berk's life and kept her
behind bars for a full decade, and the fact that this analysis
targeted and harmed just one person makes it... Continue Reading
If you need to assess process
performance relative to some specification limit(s),
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 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
Any time you see a process
changing, it's important to determine why. Is it indicative of a
long term trend, or is it a fad that you can ignore since it will
be gone shortly?
For example, in the 2014 NBA Finals, the San Antonio Spurs beat
the two-time defending champion Miami Heat by attempting more
3-pointers (23.6 per game) than any championship team in league
history. In the 2015 regular... Continue Reading
bottled water. Refreshing, convenient...and sometimes pricey. Or in
my case, I should say usually pricey. Confession: I’m a
sucker for water that comes in the “pretty” plastic bottles, and my
experience is that the pretty-bottle brands are usually the pricier
ones. Does bottled water cost increase with the fanciness of the
bottle? Well, that could be an analysis for a different day …
My... Continue Reading
If you perform linear regression analysis, you might need to
compare different regression lines to see if their constants and
slope coefficients are different. Imagine there is an established
relationship between X and Y. Now, suppose you want to determine
whether that relationship has changed. Perhaps there is a new
context, process, or some other qualitative change, and you want to
determine... Continue Reading
In an earlier post, I shared an
overview of acceptance sampling, a method that lets you
evaluate a sample of items from a larger batch of products (for
instance, electronics components you've sourced from a new
supplier) and use that sample to decide whether or not you should
accept or reject the entire shipment.
There are two approaches to acceptance sampling. If you do it by
attributes, you... Continue Reading
In my last post, I showed how to use Minitab Statistical
create an acceptance sampling plan by variables, using the
scenario of a an electronics company that receives monthly
shipments of LEDs that must have soldering leads that are at least
2 cm long. This time, we'll compare that plan with some other
The variables sampling plan we came up with to verify the... Continue Reading
If you're just getting started in the world of quality
improvement, or if you find yourself in a position where you
suddenly need to evaluate the quality of incoming or outgoing
products from your company, you may have encountered the term
"acceptance sampling." It's a statistical method for evaluating the
quality of a large batch of materials from a small sample of items,
This is an era of massive data. A huge amount of data is being
generated from the web, from customer relations records but also
from sensors used in the manufacturing industry (semiconductor,
pharmaceutical, petrochemical companies and many other
Univariate Control charts
In the manufacturing industry, critical product characteristics
get routinely collected to ensure that all products... Continue Reading
of us have data stored in a database or file that we need to
analyze on a regular basis. If you're in that situation and you're
using Minitab Statistical Software, here's how you can save some
time and effort by automating the process.
When you're finished, instead of using File > Query
Database (ODBC) each time you want to perform analysis on
the most up-to-date set of data, you can add a... Continue Reading
There are many reasons why a distribution might not be
normal/Gaussian. A non-normal pattern might be caused by several
distributions being mixed together, or by a drift in time, or by
one or several outliers, or by an asymmetrical behavior, some
out-of-control points, etc.
I recently collected the scores of three different teams (the
Blue team, the Yellow team and the Pink team) after a laser... Continue Reading
Having delivered training courses on
capability analyses with Minitab, several times, I have noticed
that one question you can be absolutely sure will be asked, during
the course, is: What is the difference between the Cpk
and the Ppk indices?
Ppk vs. Cpk indices
The terms Cpk and Ppk are often confused,
so that when quality or process engineers refer to the
Cpk index, they often actually intend to... Continue Reading
Back when I was an undergrad in
statistics, I unfortunately spent an entire semester of my life
taking a class, diligently crunching numbers with my TI-82, before
realizing 1) that I was actually in an Analysis of Variance (ANOVA)
class, 2) why I would want to use such a tool in the first place,
and 3) that ANOVA doesn’t necessarily tell you a thing about
Fortunately, I've had a lot more... Continue Reading
Control charts are a fantastic tool. These charts plot your
process data to identify common cause and special cause variation.
By identifying the different causes of variation, you can take
action on your process without over-controlling it.
Assessing the stability of a process can help you determine
whether there is a problem and identify the source of the problem.
Is the mean too high, too low,... Continue Reading
time I touched on the subject of
the greatest Super Bowl quarterback, I promised a multivariate
analysis considering several different statistics. Let’s get right
to a factor analysis.
Getting Ready for Factor Analysis
One purpose of factor analysis is to identify underlying factors
that you can’t measure directly. These factors explain the
variation of many different variables in fewer... Continue Reading
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