About four inches

The answer was 'about four inches' - what was the question?

Once you have finished sniggering in the back row, the real answer is an unreal pretence of accuracy.

This mini-rant was inspired by a weather forecast, heard on the radio a few days ago for some eastern part of the UK or other. We were told that 100 millimetres of rain was expected 'which is about four inches.'

Now it is perfectly reasonable to say that 100 millimetres is about four inches, as it is actually pretty close to 4.16 inches. But the point is that there weren't really going to be 100 mm of rain.

In reality that '100 mm' number was just a round figure guess. There was no significant accuracy to the value. So the inches version should be a round figure too - in this case, four inches, not 'about four inches'. Otherwise it suggests a totally spurious accuracy in the original 100 millimetres.

When I worked for a certain large airline with the initials BA, we used to have a similar problem with the people involved in scheduling aircraft. The planning system included various variables, like passenger load, and we worked out the weight of the total passengers on board (because that influences the amount of fuel you need) using an average figure. One of our planners wanted us to change the system so he could put in passenger weights to two decimal places. But given this was a vague estimate, such accuracy was worse than meaningless: it gave the figures a spurious reality.

And that's why I'm picking up the weather forecaster highly unfairly on what was actually a very natural thing to say. It's just too easy to give a forecast figure, which in the end is an informed guess, a spurious sense of accuracy, and we need to be on our guard to avoid this.

Image from Wikipedia


  1. It is sad that most people, even after nominally passing some studies of mathematics and physics in high school, are unable to differentiate between the concepts of precision and accuracy. They tend to add precision when they should try to have more accuracy.

    Far too often I see this in my own language (Finnish), in subtitles of TV series or movies translated from American English: "the place is about 169 kilometres from here" when the original speaks of "about a hundred miles".

    I think they shouldn't even convert from American/imperial units to metric when translating. People might actually learn something, for instance that an acre is a tenth of a square furlong.

  2. Thanks, PJT - good point. Though I'm not sure how many people use square furlongs!


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