Annual Incomes of 10 Families

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Why do we need the standard deviation? 1- The
standard deviation reflects dispersion of data
values, so that the dispersion of different
distributions may be compared by using standard
deviations. 2- The standard deviation permits
the precise interpretation of data values within
a distributions. 3- The standard deviation, like
the mean, is a member od a mathematical system
which permits its use in more advanced
statistical considerations.
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EMPIRICAL RULES 1- About 68 of the values will
lie within 1 standard deviation of the mean, that
is, between x - s and x s 2- About 95 of
the values will lie within 2 standard deviation
of the mean, that is, between x - 2s and x
2s 3- About 99.7 of the values will lie within
3 standard deviation of the mean, that is,
between x - 3s and x 3s
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