3.27 Perception/decoding: Visual tasks

  • “Beyond core matters of perception lies the question of interpreting and understanding particular kinds of graphs” (Healy 2018, Ch. 1.4)
  • Decoding = Perception and Interpretation involve specific visual tasks
    • Data values were encoded or mapped in to the graph, and now we have to get them back out again
  • Task: Estimate and compare (two) values within the graph
  • Interpretation/decoding has been tested in different experiments (e.g. William S Cleveland and McGill 1984b; Cleveland and McGill 1987; Heer and Bostock 2010)
  • Q: What does a viewer need to understand to decode a scatterplot?16


Cleveland, William S, and Robert McGill. 1984b. “Graphical Perception: Theory, Experimentation, and Application to the Development of Graphical Methods.” J. Am. Stat. Assoc. 79 (387): 531–54.

Cleveland, William S, and Robert McGill. 1987. “Graphical Perception: The Visual Decoding of Quantitative Information on Graphical Displays of Data.” J. R. Stat. Soc. Ser. A 150 (3): 192.

Healy, Kieran. 2018. Data Visualization: A Practical Introduction. Princeton University Press.

Heer, Jeffrey, and Michael Bostock. 2010. “Crowdsourcing Graphical Perception: Using Mechanical Turk to Assess Visualization Design.” In Proceedings of the SIGCHI Conference on Human Factors in Computing Systems, 203–12. CHI ’10. New York, NY, USA: Association for Computing Machinery.

  1. Needs to understand “what a variable is, what an x-y coordinate plane looks like, why we might want to compare two variables on one, and the convention of putting the supposed cause or”independent" variable on the x-axis. Even if the viewer understands all these things, they must still perform the visual task of interpreting the graph" (Healy 2018).