Keeping it Simple -- the Area Principle

If you made it through the [rather long] post entitled "Picturing Categorical Data," this next post brings out a fine point about making graphical displays. If you look through articles and newspapers, or perhaps at slides that people make in your office for presentations, you might see a tendency for folks to want to make them fancy. While this is admirable -- trying to show potentially dry information in a more splashy way -- it can end up being misleading. Read on.

Take a look at this pie chart, which graphs quantities that are 10%, 20%, 30%, and 40% of the whole.

Now compare it to its flashy 3-D counterpart:

"What's the difference?" you might ask. However, I would argue that the 3-D version makes the green slice (30%) look larger than the purple (40%) slice. Can you see it? This doesn't always happen with 3-D displays, but 3-D displays are prone to this. It's something that you want to look out for, just in case.

When a smaller pie slice or bar (in a bar chart) looks larger than a slice or bar that represents a larger quantity, we say that the display violates the Area Principle. In the first display, each piece was proportionately sized relative to the others.

Why make this point? When trying to communicate something graphically, the main point is to get the information across with as little potential confusion as possible -- not to impress people with fancy pictures. Statistics can be mind-boggling to many, so why not try to make things as straightforward as possible? It's the old K.I.S.S. principle. Statistics-challenged people will thank you! (As I'm sure you're thanking me for the short post!)