Tuesday, March 25, 2014

A Turducken Infographic from the Idaho Statesman

The front page of last Sunday’s Idaho Statesman had an article titled Idaho colleges aim for higher tuition, again with a curious infographic. It both tabulated and plotted how tuition at Boise State University increased 79 percent from $3,520 in 2003 to $6,292 in 2013. That data is shown above in a conventional line chart, with the y-axis starting from zero, which is an honest way of plotting.

The Statesman decided white space on their front page was too valuable to waste, so they instead cut off the bottom of their graphic just below $3000. Why did I call it a turducken? A turducken is a 3-in-1 deboned fowl mashup consisting of a chicken stuffed inside a duck, stuffed inside a turkey.    

Their graphic plotted the data as a blue line chart, with the value for each blue-circled  data point labeled beside it. Beneath the values for even-numbered years are orange bars, and beneath the values for odd-numbered years are tan bars.

So, first their graphic is equivalent to a table (the chicken). Second, it’s a tiger-striped bar chart (the duck). Third, it’s a line chart (the turkey). It’s clever, but somewhat misleading.

The Misleading Graph page at Wikipedia says that one with a Y-axis that doesn’t start at zero is a truncated or torn graph.

On page 56 of Edward Tufte’s book The Visual Display of Quantitative Information (2nd edition, 2001) he states two principles of graphical integrity:

“The representation of numbers, as physically measured on the surface of the graphic itself, should be directly proportional to the numerical quantities represented.

Clear, detailed, and thorough labeling should be used to defeat graphical distortions  and ambiguity. Write out explanations of the data on the graphic itself. Label important events in the data.”

On the next page Tufte defines a Lie Factor:

“Lie Factor = (size of effect shown in graphic)/(size of effect in data)”

and says if the Lie Factor is less than 0.95 or greater than 1.05 the graphic isn’t accurately representing the underlying numbers. The Stateman’s truncated graphic has a Lie Factor of about 0.57, which is pretty misleading.

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