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# S1 question help??

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Why bother with a post grad? Are they even worth it? Have your say! 26-10-2016
1. Hi, can anyone help me with explaining how this works? I really don't understand why they've taken the product of the sample size and the standard deviation away from the mean...? thanks
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2. They haven't taken the product of the sample size and the standard deviation away from the mean. With a normal distribution 68.2% of results will be within +/- one sigma, 95.4% will be within +/- two sigma and 99.7% will be within +/- 3 sigma. What they are showing is that with an average volume of 118 L and a standard deviation of 65 L this only allows for -1.82 sigma until the volume is at zero. So, as you can't have a negative value for the volume, the distribution must be skewed to have all of the measurements above zero.

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