Friday, September 19, 2008

Do time series charts really compare time series?

Selecting the best way to chart time data ...

"You can find time series comparisons in almost every newspaper, business journal and political magazine. Although they are frequently used in the media, these charts can be very misleading. What you should know about developments over time so that you don’t jump to the wrong conclusions.

Recently, it seems that I experience shocks like these quite often. Take the chart below. The lines suggest that the price for heating oil has exploded, while electricity and natural gas have increased moderately.

If you do the math yourself, however, you can also see that the blue line for natural gas has grown at about twice the rate as the green line for electricity and almost at the rate of the red line for heating oil. Apart from general inflation and the increase in value added tax over that period – honestly…do you see that?

 - Usability, User Interface Design

I don’t. As I recently sat with my son over a few geometry problems, I suddenly realized why this is. We observed two lines that run parallel to each other. We asked ourselves: Is ‘parallel’ equivalent to ‘similar’ – and learned that lines run parallel to each other when they have the same absolute slope. This is the case when the absolute changes per period have the same value and sign. Where they start is irrelevant."    (Continued via Me, myself and BI)    [Usability Resources]

Data Over Time - Usability, User Interface Design

Data Over Time


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