By Jock Mackinlay 2008/08/05
The Junk Charts blog had a posting about web publishing that included a comment by reader 'DavidS'. He suggested using quartiles in a chart about life expectancy in OECD countries. Since the data was easy to download, I explored his suggestion in Tableau. Although the original chart is a good summary of the upward trend of life expectancy, David is correct that a more statistical view showing outliers demonstrates that the variance has increased even though the range has reduced.

Since I was creating a statistical view, I decided to use a density plot to show the distributions by year. This also allows you to see when data was not collected for a given country.

Life Expectancy

And since there were more countries than an effective nominal color pallet, I used distinct color hues only for the low outlier countries. In Tableau, you can use the highlight feature of the color legend to explore the data for individual countries. The static view clearly shows that some outliers have moved significantly toward the mean.

As David predicted, the quartile markings (i.e., the small black lines) all show that the variance has increased even though the range has reduced.

You can explore further with the free Tableau Reader application (details / direct download) and the attached Tableau Packaged Workbook, which also includes trend line and map views of this data. JunkChart's original blog entry is available here and you can find David's comment here.