Common Mistakes made in Chapter 1 – 6
#1 – Marginal distribution is always out of the TOTAL
and only uses the #’s in the MARGINS not the body of
the table.
females
males
TOTALS
cats
25
6
31
dogs
15
30
45
TOTALS
40
36
76
#2 – The mean is pulled in the direction of the
skewness.
This is skewed right so
MEAN > MEDIAN
#3 – Segmented Bar graphs must all be 100% tall. If
the segments areas are EQUAL then the variables are
independent.
#4 – When discussing DISTRIBUTION talk about:
SHAPE:
Unimodal, Bimodal, Multimodal, Symmetric, Skewed Left,
Skewed Right, Uniform
CENTER: median, mean
SPREAD:
Range, IQR, Standard Deviation
UNUSUAL CHARACTERISTICS: gaps, outliers
#5
 When data is skewed or has outliers use Median and
IQR.
 When data is symmetric and unimodal use mean and
standard deviation ( or range)
#5 – Know how to find OUTLIER FENCES:
Left Fence: Q1 – 1.5IQR
Right Fence: Q3 + 1.5IQR
 #’s that fall outside the fences are considered outliers
# 6 – Measures of Position: 5 # summary and Mean
Measures of Spread:
Adding/Subtracting(SHIFT)
Multiplying/Dividing
(RESCALING)
Range, IQR, Standard Deviation
Measures of Position
CHANGE amount you + or CHANGE amount you • or /
Measures of Spread
DON”T CHANGE
CHANGE amount you • or /
Histograms are for QUANTITATIVE VARIABLES ( involve units)
Bar Graphs are for CATEGORICAL VARIABLES ( group according to
trait)