Raw numbers are easy to report and analyze, but without the proper context, they can be misleading. Is the effect you’re seeing real, or a simple result of the underlying, obvious distribution? Too many analyses and news stories end up reporting things we already know. Continue reading Putting Data Into Context
Percentile Bandsfor Unemployment Data in 380 U.S. Metro Areas
The visualization above shows the unemployment rate in 380 metro areas in the U.S. from January 2003 to June 2013 (data from the Bureau of Labor Statistics). Each of these is itself an average, but the overall mean is also shown as a heavier line. Mouse over to see individual metro areas highlighted.
As you explore, you will see many small and large patterns that the average, or mean, completely misses. You can see some outliers with very high unemployment, Hurricane Katrina, seemingly random spikes, etc. (click these links to highlight them in the visualization above, click again to turn the highlight off). That is part of the function of the mean: it averages away small changes. That can be a desired effect, but it is often glossed over when numbers like unemployment rates are reported. Does a small change in the average taken over 200 million people really mean much? Worse yet, does no change mean that nothing happened?
How do you account for the large variation in this data, though? One way is to include a range based on percentiles. The most obvious would be to report the range from smallest to largest value. That does tend to be very sensitive to outliers, however, which may or may not be desirable. Instead, perhaps a narrower range should be reported that covers most of the data, with the extreme values treated separately. But which one?
Percentiles are one of the simplest ideas in statistics: sort the data values, then pick the ones you want depending on their location in that list (as a fraction of the length of the list). The value in the middle is the 50th percentile, also known as the median. The value one quarter of the way into the list is the 25th percentile, etc. Picking the range of values from the 25th to the 75th percentile selects half the data (dropping the bottom and top quarters); this is also called the interquartile range.
A common way of looking at data is to drop the top and bottom 5%, which leaves the range from 5% to 95% (clicking these links will change the settings of the visualization above). That removes quite a bit of the range, though. Is 1% to 99% better? How about the interquartile range? Talking about percentiles in the abstract is one thing, but seeing how much data, and how much of the range of values, that ignores, is quite another.
Calculating percentiles requires additional data. With unemployment data, there is some on metro areas, sectors, and a number of demographic values. In other cases, that data is often not easy to find or simply not available. But whenever possible, we need to demand more context than a single number. A simple mean without such context is meaningless.
Colors are perhaps the visual property that people most often misuse in visualization without being aware of it. Variations of the rainbow colormap are very popular, and at the same time the most problematic and misleading. Continue reading How The Rainbow Color Map Misleads
The same data can look very different in a line chart depending on its aspect ratio. But what is the perfect shape for a chart? A square? A rectangle? Which rectangle? It depends on the data. Continue reading Aspect Ratio and Banking to 45 Degrees
One of the most common mistakes people make when creating charts is to cut off the vertical axis. But why is that a problem? And what can you do when you need to show data where the amount of change is small compared to the absolute values? Continue reading Continuous Values and Baselines
Data comes in a number of different types, which determine what kinds of mapping can be used for them. The most basic distinction is that between continuous (or quantitative) and categorical data, which has a profound impact on the types of visualizations that can be used. Continue reading Data: Continuous vs. Categorical