Three years ago I wrote a short piece on how the FT had misrepresented the ratings of President Obama by using an inappropriate vertical scale on a graph it was showing: note that piece was on another of my blogs!

My simple suggestion was to elongate the vertical scale and reinterpret the graph.

I just reviewed that posting and feel I ought to have suggested that we calculate rates of change values for the ratings rather than just change the chart. That will give us a complete picture.

Here’s the original graph:

obama_rate_wrong

Here is my table of data: assumed values from the chart then calculated values of the rates of change. Finally my own graph showing both sets of numbers: look at the impact of the rates of change values!

Year

Month

Change %

Rate of
Change %

2009

Feb

66%

2009

Mar

63%

-4.55%

2009

Apr

61%

-3.17%

2009

May

64%

4.92%

2009

Jun

64%

-0.78%

2009

Jul

60%

-5.51%

2009

Aug

55%

-8.33%

2009

Sep

52%

-5.45%

2009

Oct

52%

0.00%

2009

Nov

52%

0.00%

2009

Dec

50%

-3.85%

2010

Jan

51%

2.00%

image

Sorry the example is so old but it’s a good one because the rates of change are at such apparent odds with the way the chart was originally presented.

Duncan Williamson

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