Blog posts and articles about the role of the normal distribution in statistics, data analysis, and quality improvement.

Scientists who use the Hubble Space Telescope to explore the
galaxy receive a stream of digitized images in the form binary
code. In this state, the information is essentially worthless-
these 1s and 0s must first be converted into pictures before the
scientists can learn anything from them.
The same is true of statistical distributions and parameters that are used to describe sample data. They... Continue Reading

In
previous posts, I discussed the results of a recycling project done
by Six Sigma students at Rose-Hulman Institute of Technology last
spring. (If you’re playing catch up, you can read Part I and Part II.)
The students did an awesome job reducing the amount of recycling
that was thrown into the normal trash cans across all of the
institution’s academic buildings. At the end of the spring... Continue Reading

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by Matthew Barsalou, guest
blogger.
The old saying “if it walks like a duck, quacks like a duck and
looks like a duck, then it must be a duck” may be appropriate in
bird watching; however, the same idea can’t be applied when
observing a statistical distribution. The dedicated ornithologist
is often armed with binoculars and a field guide to the local birds
and this should be sufficient. A... Continue Reading

As a Minitab
trainer, one of the most common questions I get from training
participants is "what should I do when my data isn’t normal?" A
large number of statistical tests are based on the assumption of
normality, so not having data that is normally distributed
typically instills a lot of fear.
Many practitioners suggest that if your data are not normal, you
should do a nonparametric version of... Continue Reading

A while back, I offered an
overview of process capability analysis that emphasized
the importance of matching your analysis to the distribution of
your data.
If you're already familiar with different types of
distributions, Minitab makes it easy to identify what type of data
you're working with, or to transform your data to approximate the
normal distribution.
But what if you're not so great with... Continue Reading

In 1898, Russian economist Ladislaus Bortkiewicz published his
first statistics book entitled Das Gesetz der keinem
Zahlen, in which he included an example that
eventually became famous for illustrating the Poisson distribution.
Bortkiewicz researched
the annual deaths by horse kicks in the Prussian Army from
1875-1984. Data was recorded from 14 different army corps, with one
being the Guard... Continue Reading

To choose the
right statistical analysis, you need to know the distribution of
your data. Suppose you want to assess the capability of your
process. If you conduct an analysis that assumes the data follow a
normal distribution when, in fact, the data are nonnormal, your
results will be inaccurate. To avoid this costly error, you must
determine the distribution of your data.
So, how do you determine... Continue Reading

Imagine that you are watching a race and that you are located
close to the finish line. When the first and fastest runners
complete the race, the differences in times between them will
probably be quite small.
Now wait until the last runners arrive and consider their
finishing times. For these slowest runners, the differences in
completion times will be extremely large. This is due to the fact
that... Continue Reading

by Lion "Ari" Ondiappan Arivazhagan, guest blogger.
Predicting project completion times is one of the major
challenges project managers face. Project schedule overruns are
quite common due to the high uncertainty in estimating the amount
of time activities require, a lack of historical data
about project
completion, organizational culture, inadequate skills, the complex
and elaborative nature of... Continue Reading

Welcome to the Hypothesis Test Casino! The featured game of the
house is roulette. But this is no ordinary game of
roulette. This is p-value roulette!
Here’s how it works: We have two roulette wheels, the Null wheel
and the Alternative wheel. Each wheel has 20 slots (instead of the
usual 37 or 38). You get to bet on one slot.
What happens if the ball lands in the slot you bet on? Well,
that depends... Continue Reading

It’s safe to say that most people who use statistics are more
familiar with parametric analyses than nonparametric analyses.
Nonparametric tests are also called distribution-free tests because
they don’t assume that your data follow a specific
distribution.
You may have heard that you should use nonparametric tests when
your data don’t meet the assumptions of the parametric test,
especially the... Continue Reading

In part 1 of this post, I covered how Six Sigma students at
Rose-Hulman Institute of Technology cleaned up and
prepared project data for a regression analysis. Now we're
ready to start our analysis. We’ll detail the steps in that process
and what we can learn from our results.
What Factors Are Important?
We collected data about 11 factors we believe could be
significant:
Whether the date of... Continue Reading

By Peter Olejnik, guest blogger.
Previous posts on the Minitab Blog have discussed the work of
the Six Sigma students at Rose-Hulman Institute of Technology
to reduce the quantities of recyclables that wind up in the trash.
Led by Dr. Diane Evans, these students continue to make an
important impact on their community.
As
with any Six Sigma process, the results of the work need to be
evaluated. A... Continue Reading

If you wanted to figure out the probability that your favorite
football team will win their next game, how would you do it?
My colleague
Eduardo Santiago and I recently looked at this question, and in
this post we'll share how we approached the solution. Let’s start
by breaking down this problem:
There are only two possible outcomes: your favorite team wins,
or they lose. Ties are a possibility,... Continue Reading

by Matthew Barsalou, guest
blogger.
E. E. Doc Smith, one of the greatest authors ever, wrote
many classic books such as The Skylark of Space and
his Lensman series. Doc Smith’s imagination knew no
limits; his Galactic Patrol had millions of combat fleets under its command
and possessed planets turned into movable, armored weapons
platforms. Some of the Galactic Patrol’s weapons may be well... Continue Reading

In technical support, we frequently receive calls from Minitab
users who have questions about the differences between Cpk and
Ppk.
Michelle Paret already wrote a great post about the
differences between Cpk and Ppk, but it also helps to have a
better understanding of the math behind these numbers. So in this
post I will show you how to calculate Ppk using Minitab’s default
settings when the... Continue Reading

Have you ever had a probability
plot that looks like this?
The probability plot above is based on patient weight (in
pounds) after surgery minus patient weight (again, in pounds)
before surgery.
The red line appears to go through the data, indicating a
good fit to the Normal, but there are clusters of plotting
points at the same measured value. This occurs on a probability
plot when there are many... Continue Reading

Last
week, thanks to the collective effort from many people, we held
very successful events in Guadalajara and Mexico City, which gave
us a unique opportunity to meet with over 300 Spanish-speaking
Minitab users. They represented many different industries,
including automotive, textile, pharmaceutical, medical devices, oil
and gas, electronics, and mining, as well as academic institutions
and... 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

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