Wednesday, October 29, 2008

Bad map. Bad bad map.

I came across this map of African-American population that unintentionally illustrates some bad practices:


First off, the map uses confusing symbols. The colors are supposed to illustrate the percentage of Black population by county. You aren't told that the breakdown is by county - you just figure that out after looking at the less-populated areas of the country. Which brings up point number two: using large dots which actually obscure much of the map, and obscure each other. I can get an overall impression of Black distribution across the country, but I can't examine any of the Southern states to get a more precise look at the demographics - the symbols obscure more than they reveal. The large dots are meaningless in this context, which is error number three. They may be proportional to actual population - in fact, that seems the most likely explanation - but there's no information about that at all.

Bad practice number four isn't all that awful, but it's still worth mentioning: the categories break at very odd percentage values. I mean, really - 0.09% instead of 0.10%? What's happened is that the categories have been broken down along strict quintiles - the lowest 1/5 of percentages happens to top out at 0.09, so that's the breakpoint. Better practice, though, is to look for either natural break points (if the data tends to cluster very much) or just fudge to rounder numbers so you don't give a false sense of meaning to the category boundaries.

And finally, those colors. Given the association of red/yellow/green with danger/caution/safety in our culture, what can the map creator possibly have been thinking in assigning this color scheme to such a map? Ai yi yi yi yi.

5 comments:

Heather said...

Heh. You're right, the colors fairly scream "stay away from the South!" Though honestly, that's something I generally try to do anyway...

You should post more map criticism. I love maps, and I'm surprisingly pleased by map-related snark.

James Hanley said...

So color represents density, but he also uses marker size to represent density? Do I understand that correctly? It seems an astonishingly odd thing to do.

But I notice the mapmaker is from CSUN. As my friend, Blake, earned his geography degree at CSUN, I'll eagerly use this to mock the quality of his education (my degree is from CSUB--a world of difference, you know).

Scott Hanley said...

The dot size seems to be trying to represent population size in some manner, but I'm not sure exactly how. Maybe actual raw population figures, by county? Population density (i.e., people/square mile)? I dunno.

Anonymous said...
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Anonymous said...

yeah.. really like this post :))