12 April 2008

How graphs can lie to you

I want to discuss graphs for a bit. When we were wee first years at college, we spent a fair amount of time talking about how to make graphs well. Anyone can make a graph in Excel, but you need some knowledge to choose the right graphs for the right sets of data. You need to know when you need error bars and how to choose the right size error bars. Scale is an important aspect as well. Have a look at these two graphs.

This screen capture was taken Friday night, little before 7 pm central time.

The two graphs set the front runner as the whole and the other candidates as portions of that whole. No matter how many people are added, Obama and McCain stay as 100% of the available graph. But the more important issue is that it makes it look like Obama and McCain have the same amount of support, and that Hillary is way behind McCain. Thats part of the reason I started making my own graphs.

This graph is linked to the data on my google spreadsheet, it will update.

This graph puts all six of the candidates that ABC/Facebook is tracking on the same scale. Its pretty clear how huge of a lead Obama has among the Facebook crowd. Its also clear that Hillary is actually ahead of McCain by about 35,000 people, but thats rather hard to see since the difference is much smaller relative to Obama's lead.

So when you see a graph, look for the scale and just because two bars look the same doesn't mean that they're the same value.

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