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.