Monday, April 25, 2011

Statistical Rules of Thumb, part II

A while back, Will Dwinnell posted on two books, one of which is one of my favorites as well:

Will mentioned a few general topics covered in the book, but I thought I would mention two specific ones that I agree with wholeheartedly.

7.3: Always Graph the Data
In this section he quotes E.R. Tufte as follows (Abbott quoting van Belle quoting Tufte):
Graphical Excellence is that which gives the viewer the greatest number of ideas in the shortest time with the least ink in the shortest space.

I'm not so sure I agree with the superlatives, I certainly agree with the gist that excellence in graphics is parsimonious, clear, insightful, and informationally rich. Contrast this to another rule of thumb:

7.4: Never use a Pie Chart
well, that's not exactly rocket science; pie charts have lots of detractors...The only thing worse than a pie chart is a 3-D pie chart!

7.6: Stacked Barcharts are Worse than Bargraphs.
Perhaps the biggest problem with stacked bar graphs (such as the one here) is that you cannot see clearly the comparison between the colored values in the bins.



(a good summary of why they are problematic is in Stephen Few's Newletter, which you can download here)

I have found that data shown in a chart like this can be shown better in a table, perhaps with some conditional formatting (in Excel) or other color coding to push the eye toward the key differences in values. For continuous data, this often means binning a variable (akin to the histogram) and creating a cross-tab. The key is clarity--make the table so that the key information is obvious.

2 comments:

Tim Manns said...

Re: your stacked barchart. One thing I often do is instead calculate the difference as a perecentage (or 0 to 1 scale) from the mean. I use a line for each category (instead of where all your categories stack to a bar) and use a line chart (assuming your x-axis is time or some additive scale).

Basically it shows variation, which is essentially what people often try to use stack barcharts for.

Definitely agreed with you about piecharts; that made me laugh!

Lewis N. Clark said...

Graphical Excellence is that which gives the viewer the greatest number of ideas in the shortest time with the least ink in the shortest space. fengshui