"Mathematics is the Queen of Science, and Arithmetic the Queen of Mathematics"  (Carl Friedrich Gauss, 17771855)

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Reading: Pages 4346, Summary (pages 4748) Do exercises 1.35, 1.36, 1.37. 
You are in SECTION 1.2.
standard deviation
variance
degrees of freedom
Shape of data is important: ALWAYS PLOT YOUR DATA.
For distributions that are roughly symmetric:Mean and standard deviation are useful indices.
For distributions that are skewed, mean and standard deviationare less useful. The fivenumber summary presents a better overallpicture.
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Text:
The Practice of Statistics, by Yates, Moore, McCabe. New York,W.H. Freeman and Company, 1999. (ISBN 0716733706)
Supplemental books:
The Cartoon Guide to Statistics, by Gonick and Smith. NewYork, HarperCollins Publishers, 1993. (ISBN 0062731025)
How to Lie with Statistics, by Darrell Huff. New York, W.W.Norton & Company, 1982 (ISBN 039309426X)