Quote:
Originally Posted by Vekke
- Make about 20 tests so 10 A and 10 B
- If there is lots of variation between runs, even more
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In order to be scientific about this, you need to use statistics to determine the required sample size. If possible, bring a spreadsheet with you during your testing. As you increase the number of entries, (hopefully) the standard deviation will decrease and you'll be able to say with greater confidence how accurate your numbers are.
I took the 6 A and 4 B runs tabulated here:
Reflections on side mirrors: testing drag vs. MPG - MetroMPG.com
and ran it through Excel. For A runs, the mean is 55.2, with a standard deviation σ=.28. Therefore, you can say with 95% confidence (2σ) that his mpg was between 54.7 and 55.8, or with 99% confidence (3σ) that it was between 54.4 and 56.1 - you're >99% sure it was less than the 56.5 mpg that he reported in configuration B.
If you spend too much time out there, e.g. with 20 runs, other parameters like ambient temperature, driver attention, and the weight of fuel in your tank will start to drift, potentially skewing your data.