Seeing trends

Forbes used this chart to illustrate the doubling of tuition at four-year private colleges in the last two decades.

The College Board publishes annual data going back to 1976-1977 (I couldn't find the 1975-1976 data.)

The designer made two curious design choices: sampling at 5-year intervals, and log scale.  To investigate these choices, I first reversed them, producing the chart on the left below.


This chart shows the upward doubling trend very clearly so neither design choice added information.

Sampling at five-year intervals reduced the sample size from over 20 to 6.  It creates an illusion of smooth linear increases over time when in fact there have been some declines in real terms from year to year.

The introduction of a log scale is inexcusable.  With no extreme values, the raw data fits nicely onto the graph.  When the purpose of the chart is to show changes between periods, the log scale creates unwarranted distortion as a given difference at higher tuition values now represent more difference than the same physical distance at lower tuition values.

Reference: Forbes, March 13 2006

Comments

[UndergradChemist]

I know that at least in many finance circles, log charts are used in order to approximate the idea that from one period to the next, the physical change on the y scale in any period on the chart corresponds approximately to the same percentage change no matter where you're doing the measuring from.

[John S.]

Also - maybe it's my eyes, but if the y-axis is a log scale, shouldn't the distance between 25 and 30 be smaller than the distance between 20 and 25? Looks to me like they just drew in a line halfway between 20 and 30 and called it 25.