A histogram is a specialized type of bar chart. Individual
data points are grouped together in classes, so that you can get
an idea of how frequently data in each class occur in the data
set. High bars indicate more points in a class, and low bars
indicate less points. In the histogram show above, the peak is in
the 40-49 class, where there are four points.
The strength of a histogram is that it provides an
easy-to-read picture of the location and variation in a data set.
There are, however, two weaknesses of histograms that you should
bear in mind:
The first is that histograms can be manipulated to show
different pictures. If too few or too many bars are used, the
histogram can be misleading. This is an area which requires some
judgment, and perhaps some experimentation, based on the
Histograms can also obscure the time differences among data
sets. For example, if you looked at data for #births/day in the
United States in 1996, you would miss any seasonal variations,
e.g. peaks around the times of full moons. Likewise, in quality
control, a histogram of a process run tells only one part of a
long story. There is a need to keep reviewing the histograms and
control charts for consecutive process runs over an extended time
to gain useful knowledge about a process.
For histograms, the following statistics are calculated:
||The average of all the values.
||The smallest value.
||The biggest value.
||An expression of how widely spread
the values are around the mean.
||The x-axis distance between the left
and right edges of each bar in the histogram.
|Number of Classes
||The number of bars (including zero
height bars) in the histograms.
||Is the histogram
symmetrical? If so, Skewness is zero. If the left hand tail is
longer, skewness will be negative. If the right hand tail is
longer, skewness will be positive. Where skewness exists, process
capability indices are suspect. For process improvement, a good
rule of thumb is to look at the long tail of your distribution;
that is usually where quality problems lie.
||Kurtosis is a
measure of the pointiness of a distribution. The standard normal
curve has a kurtosis of zero. The Matterhorn, has negative
kurtosis, while a flatter curve would have positive kurtosis.
Positive kurtosis is usually more of a problem for quality
control, since, with "big" tails, the process may well be wider
than the spec limits.
Specification Limits and Batch Performance
Where relevant, you should display specification limits on
your histograms. The specifications include a target value, an
upper limit and a lower limit. For example, if Michael Jordan is
shooting a basketball at a hoop, his target is the middle of the
hoop. His spec limits are those points in the circle of the hoop
that will just allow the ball to bounce through the basket. If
the shot is outside spec limits, the ball doesn't go in.
When you overlay specification limits on a histogram, you can
estimate how many items are being produced which do not meet
specifications. This gives you an idea of batch performance, that
is, of how the process performed during the period that you
collected data. PathMaker calculates the actual percentage of
items in the sample that fall outside specification limits.
When you have added target, upper and lower limit lines, you
can examine your histogram to see how your process is
If the histogram shows that your process is wider than the
specification limits, then it is not presently capable of meeting
your specifications. This means the variation of the process
should be reduced.
Also, if the process is not centered on the target value, it
may need to be adjusted so that it can, on average, hit the
target value. Sometimes, the distribution of a process could fit
between the specification limits if it was centered, but spreads
across one of the limits because it is not centered. Again, the
process needs to be adjusted so that it can hit the target value
Center of a Distribution
Processes have a target value, the value that the process
should be producing, where most output of the process should
fall. The center of the distribution in a histogram should, in
most cases, fall on or near this target value. If it does not,
the process will often need to be adjusted so that the center
will hit the target value.
Spread of a Distribution
The spread, or width of a process is the distance between the
minimum and maximum measured values. If the spread of the
distribution is narrower than the specification limits, it is an
indication of small variability in the process. This is almost
always the goal, since consistency is important in most
processes. If the distribution is wider than the specification
limits the process has too much variability. The process is
generating products that do not conform to specifications, i.e.
Shape: Skewness and Kurtosis
A "normal" distribution of variation results in a specific
bell-shaped curve, with the highest point in the middle and
smoothly curving symmetrical slopes on both sides of center. The
characteristics of the standard normal distribution are tabulated
in most statistical reference works, allowing the relatively easy
estimation of areas under the curve at any point.
Many distributions are non-normal. They may be skewed, or they
may be flatter or more sharply peaked than the normal
A "skewed" distribution is one that is not symmetrical, but
rather has a long tail in one direction. If the tail extends to
the right, the curve is said to be right-skewed, or positively
skewed. If the tail extends to the left, it is negatively skewed.
PathMaker calculates the skewness of a histogram, and displays it
with the other statistics. Where skewness is present, attention
should usually be focused on the tail, which could extend beyond
the process specification limits, and where much of the potential
for improvement generally lies.
Kurtosis is also a measure of the length of the tails of a
distribution. For example, a symmetrical distribution with
positive kurtosis indicates a greater than normal proportion of
product in the tails. Negative kurtosis indicates shorter tails
than a normal distribution would have. Again, PathMaker
calculates the kurtosis of histograms.
Taken together, the values for process center, spread,
skewness and kurtosis can tell you a great deal about your
process. However, unless you have a solid statistics background,
you will probably learn more from looking at the histogram itself
than from looking at the statistics. Just remember that, where
there is data in the tails near a specification limit, chances
are that some non-conforming product is being made. If your
process is actually making 5 bad parts in every thousand, and you
are sampling 20 in every thousand, it will take some time before
you find any out-of-spec parts. There are three things you should
- keep tracking data
- get help in fitting a curve to your distribution
- make sure your sampling plan is efficient.
PathMaker can help with the first, but not (yet) with the
Distributions you may encounter
- The standard normal distribution, with its zero skewness and
- A skewed distribution, with one tail longer than the other.
- A double-peaked curve often means that the data actually
reflects two distinct processes with different centers. You will
need to distinguish between the two processes to get a clear view
of what is really happening in either individual process.
- A truncated curve, with the peak at or near the edge while
trailing gently off to the other side, often means that part of
the distribution has been removed through screening, 100%
inspection, or review. These efforts are usually costly and make
good candidates for improvement efforts.
- A plateau-like curve often means that the process is
ill-defined to those doing the work, which leaves everyone on
their own. Since everyone handles the process differently, there
are many different measurements with none standing out. The
solution here is to clearly define an efficient process.
- Outliers in a histogram – bars that are removed from
the others by at least the width of one bar – sometimes
indicate that perhaps a separate process is included, but one
that doesn't happen all the time. It may also indicate that
special causes of variation are present in the process and should
be investigated, though if the process is in control before the
histogram is made as it should be, this latter option is
Create Histograms using PathMaker's Data Analyst tool.