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Decimal places
I see people here often quoting data to many decimal places. Sometimes, even (as Aerohead did recently), increasing the number of decimal places after doing a calculation.
So what's the issue?
The number of decimal places is indicative of the accuracy of the measurement, with the more decimal places, the higher the degree of assumed accuracy.
Two points.
1. You cannot increase the number of decimal places that was present in the original measurement. So for example, a 9 per cent reduction in a drag coefficient of 0.32 cannot become 0.2912 - there's no basic of validity for the last two decimal places (ie there was no such resolution in the original measurement) and so it becomes 0.29.
2. As textbook I have says of the use of too many decimal places: "They imply a very precise result from imprecise data." Therefore, the number of decimal places should reflect the uncertainty in the original measurements. If I do fuel economy measurements over a relatively short distance (i.e. not thousands of km) and get 3.2 litres/100 km, and then make a change and get 2.9 litres/100km, the improvement is 9.375 percent. But realistically, taking into account the uncertainties involved, it's better to say "about 10 per cent".
As soon as someone starts using lots of decimal places, you know they either have incredibly precise measurements - or they don't have a good feel for the data.
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