As I mature (and boy, is aging a high price to pay for maturity), I find I have very little need or even desire to win an (never mind every) argument, or to prove that I’m right about something.
I suppose that’s true in part because I understand that we all see the world in different ways, and in part because it seems to take a very long time for even solid, compelling evidence about anything to persuade people to change their firmly held beliefs. (And I admit that sometimes I count myself among those folks.)
It’s also why I’ve written very rarely (even though I’m occasionally tempted to say something) on “why I’m a card-carrying member of the ‘Better not use pie charts’ club.”
There are many expert voices, and there is plenty of evidence, on this topic.
The data-visualization pioneer Edward Tufte said that “pie charts should never be used”; William Cleveland referred to pie charts as “pop charts” because they are commonly found in pop culture media rather than in science and technology writing. Data-visualization expert Stephen Few wrote the widely-read and frequently-referenced essay “Save the Pies for Dessert.” All the same, I feel the need to add my voice to the chorus in the hope of improving healthcare data visualizations.
What pushed me over the edge?
A free e-book from a software vendor (that should have been my first clue) which, in spite of well-established expert opinions and evidence about why pie charts are not as effective as other display devices, presents advice about the misuse of pie charts – that is, it explains how to use pie charts correctly. And around we go again – oh, my aching head!
Let’s walk through what is suggested and why those suggestions constitute bad advice; and then let’s turn to the part left out: how to display data better with nary a pie chart in sight.
Here are some excerpts (I’m paraphrasing):
The book says…
“Don’t squeeze too much information into a pie chart: the slivers get too thin, and the audience confused.”
Use a bar chart like the one in Example 1, below. We humans find it very difficult to judge the size of the angle in a pie chart. With a bar chart, we can immediately tell the size of the data being encoded by the length of each bar. It’s then easy to directly compare the lengths of the bars, and determine which values are larger or smaller.
We can also add a comparison or target line if we need to, which we can’t do on a pie chart.
We can label each value being displayed directly rather than making our viewers match a color-coded key to each slice of the pie, all while trying to hold the information in short-term memory as they look back and forth from the chart to the key. (Try it, and you’ll see what I mean!)
The book says…
“Order your slices from largest to smallest for easiest comparison.”
Okay, this is just silly!! Simply use a ranked bar chart like the one in the Example 2, below.
The book says…
“Avoid using pie charts side by side – it’s an awkward way to compare data.”
Yep, you guessed it: use bar charts. And if you need to encode additional comparison data, try a bullet graph (a modified form of the bar chart). In addition to being a better way to display data, a bar chart allows additional context for visualizations.
In the example below, by using a bar chart and leveraging the fact that my viewers read from left to right, I label the data once and accomplish all of the following. I can
- show the number of cases eligible for the measure (the denominator);
- display compliance compared to target;
- note the difference between the current quarter performance and the target; and
- record how each clinician has performed over the last four quarters.
You simply can’t do all this – quickly, clearly, and in a modest display space – with a pie chart. Look at the results in Example 3, below.
Here’s the bottom line – pretty much anything you can do with a pie chart, you can do better with a bar chart. This is especially true for the types of displays we create in healthcare.
Bar charts make it easy to:
- directly compare the sizes of data groups displayed.
- directly label the data.
- easily rank the data.
- include comparison or target data.
- include additional contextual data.
As is clear from this last example above, bar charts are also far superior when used on a dashboard. They take up less space than pie charts and (as previously noted), make it possible to display much additional contextual data, such as performance over time.
Every so often I come across a forum where people still rant on about how maligned pie charts are. I admit I find them – both the people and the pie charts – infuriatingly amusing. Yes, the charts can be fun on an infographic, or useful for teaching young children the concept of part-to-whole, but for me and the work I do the evidence is in – forever – and pie charts are out.