As the old saying goes, there's more than
one way to pet a cat...and there's more
than one way to plot your data. In fact,
there must be 50 ways to plot your data.
In statistics you will probably use bar
charts and histograms the most, but here
are some examples of graphing that are
not used very often but are still useful
to know about. I'm going to show you one
chart that is very useful for count data
and another that we will use later with
correlation, plus a third that has been
called "easily the worst way to
convey information ever developed."
Pie charts show data in a circle with slices
of approximate size for each percentage.
Pie charts display proportions or
percentages, therefore the pie chart must
always add up to 100%. You
cannot use a pie chart for means or
simple counts or any data that does not
add up to one hundred percent; and you
must include all parts of the whole.
Meaning that you will probably need to
include an "other" category. So here are
some rules for pie charts: use no more
than five variables in a pie chart. Keep
it clean. The first variable should begin
at twelve o'clock and proceed from the
largest slice to the smallest in a
clockwise direction. The first largest
slice of the pie should be the darkest.
Each smaller slice is sequentially
lighter arranged from darkest to
lightest. The pie charts work best with a
small number of categories, five or fewer,
and similar size. So don't use pie charts
when one category dominates and the rest
are divided into multiple tiny slices.
You can use lines or dots
to shade the slices but you cannot use
color per the APA Style Manual 6th ed.
To create a pie chart in SPSS
using the clickers.sav dataset, go to
"Graphs" and "Chart builder." You can click
the reset button if there's still
something in the chart builder. I'm going
to use the ctrl-click or right click to
show "Display Variable Names." Now in the
gallery, choose Pie/Polar and we see
there's really only one option.
It's the pie chart. Drag the pie chart up
into the canvas and we're going to use
"Group" and drag that to the "Slice By" drop
zone. Now click "OK". There's our output.
You can double-click and use the
"Properties" values to change the fill, or
the border, or any of the settings for
this pie chart. So when should you use pie charts?
 
Well here's the thing about pie charts.
Just because you know how to create one,
doesn't mean that you should.
As U.S. President Richard Nixon once said about
paying hush money to cover up felonies,
"We could do it, but it would be wrong."
You see, there's some problems with pie
charts.
First of all, pie charts are difficult to
interpret and they tend to distort the
data, thereby displaying the data poorly.
Area is more difficult to perceive than
length, so the data will be less easily
understood than if you would simply put
the same data into a bar chart. In fact,
Edward Tufte said of pie charts, "The only
thing worse than a pie chart is lots of
them" and Tufte also tweeted, "Pie chart
users deserve the same suspicion and
skepticism as those who mix up its/it's.
there/their. To compare, use a
little table or sentences, not pies.
Generally, avoid using pie charts. As
Walter Hickey said,
"The pie chart is easily the worst
way to convey information ever developed
in the history of data visualization."
Almost every time, your best bet is going
to be to use a bar chart, and never use
the 3D option in Excel. Here are four
equally sized slices. Notice how the Red
slice appears to be much larger than the
blue even though they represent exactly
the same amount of data, and I would add
to Tufte and Hickey that, like actual pie,
pie charts are not informationally
nutritious. I would never recommend using
one in a publication, but they do
have some limited utility in the simple
display of data. For instance, here is a
pie chart that I created last
Thanksgiving. I do like that one. A Pareto
chart is a type of bar chart that arranges
the data in descending frequencies,
high-to-low. It's often used with nominal
data to make comparisons between the
items easier. In this case, we have the
GDP for a number of countries. It's easy
to see that the U.S. is the highest and
Japan is second, but who's third, fourth,
and fifth? It sure would be nice if we could
just put them in descending order.
Yeah, that is much better. To create a
Pareto chart in SPSS, we will need to
switch to a different dataset. We'll be
using the dataset Deaths in Shakespeare.sav
Go to Analyze, Quality Control, Pareto Charts.
click "Simple" and "Values of
Individual Cases" and then "Define."
We're going to move the "Number of Deaths" into
the "Values" box and "Title of the play"
will go under "Category Labels."
You should uncheck display cumulative line now click OK and let's see what that
Pareto chart looks like. Now we can see
the number of deaths per play in
descending order. The bloodiest
Shakespearean play is Titus Andronicus
followed by King Lear and then Hamlet. In
each case, you can see the actual number
of deaths in each play. A lot of people
don't know this, but this is also called
a bard graph. There's another type of
graph called a scatterplot that uses
dots. A scatterplot is a type of graph
used for correlation and regression.
Scatterplots are used to show the
relationship between two variables, not
frequency as with a frequency polygon. A
pair of scale variables, X and Y, are
plotted on their respective axes. A
single straight line called a regression
line runs through the middle of the data
points to demonstrate the strength and
the direction of the relationship
between the variables. We will learn
about scatterplots when we study
correlation.
