When you know how to create a histogram, you move from knowing
a little bit about a process to knowing a lot. When you know your
process, you've gone a long way towards making big improvements.
Learn how to make a histogram in this short article, and discover
hidden information about your process.
Why bother Creating a Histogram?
If creating a histogram is too close to a middle school math lesson
for comfort, bear with it for a while longer. It's worth the effort.
Imagine you manage a call centre - wouldn't it be neat to know when
you're customers are most likely to call and increase your staffing
to cope with the peaks, and give staff breaks during the quiet times?
Or if you produce widgets, wouldn't it be great to get all of them
out the door instead of creating piles of rejects, or worse, customer
What Does A Histogram Do?
Transforms unwieldy data into easily understood visual information
Quickly illustrates the underlying distribution of the data.
Provides insight to predict the future performance of the process
Answers the question - "can this process meet my customers'
How To Create A Histogram
Decide on the process measure - is it time, size, weight, speed
that you're going to measure? Or something like "marks out of
100" for an exam result?
Collect at least 50 to 100 data points. Any fewer and you will
have less detailed information, and any more and it might be too
time consuming or costly to collect.
Consider the time-frame that you're measuring - does it fairly
reflect the process? Is there a marked difference between shifts,
days, months? If you can't get "fresh" data, take a look at some
historical data to give you a baseline for your process.
Prepare a "distribution frequency histogram table." This sounds
scarier and harder than it actually is, so don't fret!
Count the number of samples "n". In the example given later,
we are using the thickness, in centimetres, of a fictional widget.
Determine the range for the entire sample "R" (the smallest
value subtracted from the largest value).
Create 6-12 "class intervals", with a "class width" of range
divided by number of class intervals. (Round up the class interval
range if appropriate). Each data point can fall into one, and
only one class interval, so ensure that this is the case.
For example, you have a sample size "n" of 100.
The range is the highest number 10.7 minus the lowest number
9.0 = 1.7. You choose to have 10 class intervals. The class
width is the range 1.7 divided by number of class intervals
10 = 0.17, which can be rounded up to 0.20. Your class intervals
are therefore 9.00 to 9.19, 9.20 to 9.39, 9.40 to 9.59 etc up
to 10.80 to 10.99
Construct a frequency distribution table as below.
9.00 - 9.19
9.20 - 9.39
9.40 - 9.59
9.60 - 9.79
9.80 - 9.99
9.80 - 9.99
10.00 - 10.19
10.20 - 10.39
10.40 - 10.59
10.60 - 10.79
10.80 - 10.99
Convert the frequency distribution table into a frequency
distribution histogram, in excel or something similar.
Ask yourself some key questions and you will find out if your
process is meeting or missing your customers' needs.
What is the process mean? If the mean (average) value is way
above or below your customers' specification, you're doing something
wrong and need to correct it.
What is the variance? If the variance is very high, chances
are you will miss your customers' needs a high percentage of
the time, creating costly waste and rework. You could say your
process is "not capable" of doing the job. If the variance is
very small, maybe your process is too "good" and could be loosened
up to free up time and money - conversely, maybe your consistency
could be a selling point?
What shape is your histogram? If you have two peaks in your
data, maybe you have two processes going on, for example one
machine produces larger thickness than the second. Or maybe
the distribution is skewed in one direction, maybe telling you
that the machine thickness settings gradually work loose as
the shift goes on, producing thicker and thicker pieces.
Learn more about your process by creating a histogram. You'll get
good manager grades when you make great, measurable and sustainable
process improvements. Knowing how to create a histogram is easier
than you think!