Sine illusion makes peaks and valleys on graphs look overly variable


An article in the latest Journal of Computational and Graphical Statistics (JCGS, Vol 24, Num 4, Dec 2015, p1170)) alerted me to a fascinating misperception called the “sine illusion” that causes misinterpretation of trends in variability.  See it nicely illustrated here by vision researcher Micheal Bach.  The JGCS, Susan VanderPlas and Heike Hofmann, detail “Signs of Sine Illusion—Why We Need to Care” and provide methods to counteract its misleading effects.

If you see a scatter plot that goes up and down with seemingly large scatter at the bends, get out a ruler to get the true perspective.  That is my take home message for those like me who like to be accurate in their assessments of data.

“The illusion is explained in terms of a perceptual compromise between the vertical extent and the greater overall dimensions of the section at the turn of the sine-wave figure.”

– RH Day and EJ Stecher, “Sine of an illusion,” Perception, 20; 1991, 49–55.

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