# Real vs perceived circle area in data visualisation

I am about to visualise some data using circles on top of a map but I am concerned about the real vs perceived circle size. I've read about a bias in the perception of the area of a circle. The best article I found so far is Perceptual Scaling of Map Symbols but I am unable to google more advice.

The question: Is it common to adjust the area sizes for the perception bias? If yes, how?

Any ideas or reading recommendations are welcome.

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## migrated from stackoverflow.comJan 9 '12 at 15:35

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I was recently thinking this. Generally I've never seen any interface attemnpt to perceptually scale; for instance in Google Maps you can clearly see the location markers look bigger as you scroll out. They're not actually bigger but it seems like they should be and they are perceived as larger as you zoom out. – Ben Brocka Jan 9 '12 at 16:10
Why not use squares (or rectangles), as the article suggests? Then, you can use absolute scaling without danger of perceptual inaccuracies. – Taj Moore Jan 9 '12 at 19:21

Edward Tufte, in The Visual Display of Quantitative Information (1998, 2nd ed. 2001) - from the aricle you linked to - is completely opposed to perceptual scaling, and I am not worthy to argue with him. However, it does depend on what you are trying to do.

If you are trying to clearly represent data accurately to a user, then you should use real sizes, because otherwise you are misrepresenting the data. You may have to provide more information about the data, and ways of interpreting it to help people clearly visualise it.

If, however, you are not concered about the data accuracy, but wish to give an impression of comparative values, then it might be appropriate. But you should also then be clear about what you are doing.

Tuftes problem is that if you do not represent the data accurately, you can easily overemphasise things that you wish to - in this case, differences. If A is twice the size of B, and this relationship is critical, the A should be displayed as twice the size of B. If it is more critical that users see that A is verys significantly larger than B, it might be appropriate to emphasise this. But when people accuse you are misrepresenting the data, have an answer ready.

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It is common, as laid out and explored by the already-mentioned Edward Tufte in his various writings, but as for the "how," it is as varying and subjective as perception itself.

The real issue is that an area-based visualization is not a good way to represent data because it is not accurately understood by the average viewer. Attempting to work around this by exaggerating the scaling of the elements of the visualization is just trying to force a solution that has been shown to not work well.

If at all possible, I'd suggest avoiding area-based (and also choroplethic) representations of data unless accuracy is not important. It is "boring" to mostly stick to length-based representations of data, but boring and accurate is better than interesting and misleading.

... Unless it's a sales pitch or something, in which case you might actually intend to misrepresent the data you're showing ;)

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Thanks. The accuracy is important here, I may reconsider using circles. – daniel.sedlacek Jan 10 '12 at 17:32