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Chapter 2 Describing Distributions with Numbers
Measuring center: the mean
–
“average” value
Sigma
–
“add them all up”
Not resistant: a few outliers can make s very large.
Measuring center: the median
–
“typical” value
Median
–
midpoint of a distribution
Resistant: outliers do not matter median
If # of observations n is odd
–
count (n+1)/2 observations from any sides
If # of observations n is even
–
count (n+1)/2 observations then plus then divide by 2
The mean and median of a roughly symmetric distribution are close together. If the distribution is
exactly symmetric, the mean and median are exactly the same.
In a skewed distribution, the mean is
usually father out in the long tail than is the median.
Measuring spread: the quartiles
First quartile
–
median of the n observations left to the median, larger than 25% of all obs.
Second quartile
–
median (larger than 50% of observations)
Third quartile
–
median of the n observations right to the median, larger than 75% of all obs.
Quartiles are resistant
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 Winter '09
 RICKARD

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