Transcript Slide 1

17/07/2015
Skew
17/07/2015
Skewness
17/07/2015
Examining Skewness
1. Compare the mode, median and mean against each other
Symmetrical:
mean = median = mode
Positive skew:
mode < median < mean
Negative skew: mean < median < mode
17/07/2015
Examining Skewness
2. Use quartiles
Q1
Q2
Q3
Symmetrical
Q1 Q2
Q3
Positive skew
Q1
Q2 Q3
Negative skew
17/07/2015
Examining Skewness
3. Use a measure of skewness to calculate its value
Pearson’s Coefficient of Skewness
Skewness =
Advantages?
3 (mean – median)
Standard deviation
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Outliers/Extreme Values
It is important to identify outliers (extreme values) to make box plots
more informative
17/07/2015
Identifying outliers for drawing box plots
The whiskers are now not necessarily the minimum and maximum values
Q1
x
Q2
Q3
xx
Q1 – 1.5 x IQR
Q3 + 1.5 xIQR
Or min value
Or max value