William M. Briggs

Statistician to the Stars!

Never use bar charts! Case study #1

Social secuity disability applications

This graphic comes from the New York Times article “Social Security Disability Cases Last Longer as Backlog Rises.” It obviously intends to show how applications have increased since 1998.

This is a terrible plot.

The reason is not that you should never, with only rare exceptions, use a bar chart. They are simple to construct, but there are nearly always better alternatives.

But the evil of bar charts is well known. The reason this plot is bad has to do with the number 0. Notice that the chart starts at 0, even though it isn’t until 2 million or so that we meet our first number. The only reason that the chart starts with 0 is that it is true that you can’t have less than 0 applications. This is not a good reason. They should have started with a higher number.

Don’t think it makes a difference? Then take a look at this re-drawing:

Social secuity disability applications redrawing

I drew the same plot, but started at 2 million, a more reasonable number, and I also eliminated the distracting bars. Notice anything new?

It’s the same exact numbers, but now we see a remarkable spike in 2002, after which the number of new applications actually settles down to a steady 2.5 million or so.

Why is that spike there? I have no idea—I am not an economist—but then the New York Times reporter also doesn’t explain the huge increase. For the simple reason that he never saw it—you cannot see the spike using the bar chart. You can see it using the standard, non-sexy, scatter plot (of course, an artist at the paper could have a go at the re-drawing and make it much prettier).

There are three other plots in the New York Times’s article; each are bar plots, and each are bad in the same way. Each can be re-drawn like the one above, and each has a surprise in it. It’s an illuminating exercise that I leave to the reader.

I leave you with this question: what good stories are you missing from using bad graphics?

3 Comments

  1. The other way has merit as well. I think the way you drew it is ripe for misleading people. You can take a very small change and blow it up into a huge skyscraper increase.

  2. Administrator

    29 January 2008 at 4:12 pm

    Gunnar,

    Very true; small, insignificant changes can be made to look huge.

    That’s not the case here, however. The number has gone from a little over 2 million to 2.5 million, a 25% increase. Since the point of this graph is to show how the system is being burdened by an increasing case load, it’s better to draw a graph which emphasizes this rather larger increase.

    Thanks,

    Briggs

  3. I agree with comment 1 and have often seen people playing games do exactly this sin (in business) Sometimes, still using barcharts, but clipping the axis, to make it look dramatic. Zelazny in his book on charts advies against clipping axes like this. I do agree, that one would not see the dramaiktc hop without blowing up the data, though.

Comments are closed.

© 2014 William M. Briggs

Theme by Anders NorenUp ↑