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resolution
Another point is the resolution of 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".
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If you read 3.2, you are really reading 3.2 +/- 0.05, and 2.9 +/- 0.05. While the reading says 3.2, it could actually be 3.24999, or it could be 3.1500. So your change could be anywhere between 6.4% and 12.4%, (if you assume 3.25 and 2.95, and compare to 3.25 and 2.85) so the margin of error on your 9% is plus minus 33%.
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