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Monday, June 06, 2005

When is a pie chart worthless?

When the sample size is 16?

Here's the chart in question, from the Fourth Quarter 2004 Commercial Space Transportation report.

The pie chart suggests that 6% of the space shots in the fourth quarter of 2004 were failures. Granted, 1/16 is approximately 6%. But what is the pie chart really supposed to convey? To me, the chart implies that there would be 6 failures if there had been 100 space shots rather than just 16. If the kid with the excel spreadsheet had just put 1 and 15 (and left out the percentages) I would have less of a problem with the graph. However, the kid did include the percentages and therefore the graph is basically worthless.

For this reason, I don't like the use of percentages when the sample size is considerably less than 100. If we look at historical data from the United States, from 1996-2002 the U.S. had 160 earth orbit space shots and had a total of 5 failures (1 in 1996 and 4 in 1999) for a failure rate less than 2%. Here's a pie chart of earth orbit space shots since 1996:

Is the original pie chart intended to convey that there was a higher failure rate than historical norms? Perhaps, but there is an integer problem: one failure becomes 6% but there couldn't have been a 2% failure rate. In my mind, graphs such as these just take up space but don't convey much information.

I know it isn't that important, but it still drives me nuts.

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