Misleading graphic on oil spills

Pie Are Square; Oil Spills Are Round
[Via Cosmic Variance]

Ah, not this one again. The folks at Iglu Cruises have put together a helpful infographic to explain various features of the Gulf of Mexico oil spill (via Deep Sea News). Here’s the bit where they compare the recent spill (which, by the way, is still ongoing at a fantastic rate) to previous oil spills. Click for full resolution.

Oil spills: diameter vs. area

Doesn’t make the current fiasco seem so bad, does it? That little blob on the left looks a lot smaller than the blob right next to it, representing Saddam Hussein’s dump of oil into the Persian Gulf during the first Gulf War. In fact, when you think about it, it looks a lot smaller. Which is weird, when you look at the numbers and see that the current spill is 38 million gallons (as of May 27), while the Iraqi spill was 520 million gallons, a factor of about 14 times bigger. The blob representing Iraq’s spill seems a lot more than 14 times the size of the blob for the current spill. You don’t think — no, they couldn’t have done that. Could they?

Yes, they did. When measure the diameter of the circle representing the Iraqi spill, I get about 360 pixels (in the high-res version), while the smaller spill is about 26 pixels — a factor of about 14 larger. But that’s the diameter, not the area. The area of a circle, as many of us learned when we were little, is proportional to the square of its radius: A = π r2. The radius is just half the diameter, so the area is proportional to the diameter squared, not to the diameter. In other words, that big blob is about (14)2 = 196 times the area of the little one, when it should be only 14 times bigger.

I remember reading on some other blog about this same mistake being made in a completely different context, but I have no recollection of where. (Update: it was at Good Math, Bad Math, sensibly enough.) Probably won’t be the last time.


Math idiots continue to do this – compare linear differences with a graphic displaying areas. The radius of each circle is what SHOULD be compared, but the figure makes it seem like it is the area that should be compared. Only someone mathematically illiterate, or purposefully misleading, would use such a figure.

The numbers are bad enough but improper display of data, particularly in a way that provides a a bias result, really tick me off. Check out the Good Math, Bad Math link to get a better idea of how prevalent this bad type of graphic really is (ie in financial statements all the time).

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2 Responses to “Misleading graphic on oil spills”

  1. David Bradley Says:

    I’ve re-done their graphic based on area to show approximate relative scale of the oilspills (red blobs within oily black circles). It’s not nearly so shocking as the original but mathematically a more accurate visualisation.


    • Richard Gayle Says:

      Well, done. I love seeing both so not only can one get a much better idea of the relative sizes but can also see how misleading the original was.

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