We will review last night’s homework and tonight’s
homework during next class. We will start class
immediately with notes.
CHAPTER 4
DISPLAYING QUANTITATIVE DATA
I. TYPES OF DISPLAYS
A. Dot plots – plots a ‘dot’ for each case against a
singe axis.
A. Make each dot the same size
B. Create a key if the dot stands for more than one unit.
C. Ex. How many siblings do you have?
B. STEM AND LEAF PLOTS
1. Gives a picture of the distribution while including
the actual numerical values in the graph.
2. Stem is all of the number except the final digit.
Leaf is final digit.
3. Write stems in vertical column w/ smallest at the
top (Do Not Skip any Stems!!)
4. May want to split the stems to see the distribution
better.
5. Write each leaf in the row to R of stem in
increasing order from L to R.
6. Provide a key that explains in context what the
stems and leaves represent.
B. STEM AND LEAF PLOTS
7. Example. How many pairs of shoes do you own?
Number of Shoes owned by Stat females.
50
26
26
31
57
19
24
22
23
38
13
50
13
34
23
30
49
13
15
51
EXAMPLE
Number of Shoes owned by Stat males.
14
7
6
5
12
38
8
7
10
10
10
11
4
5
22
7
5
10
35
7
Back-to-back stemplot is good for comparing two groups of the same
Variable.
C. HISTOGRAMS
1. Distribution of one quantitative variable that
divides the range of data into classes of equal
width.
2. Each bin or class includes the 1st number and goes
up to but does not include the last.
3. Y-axis can be the frequency or relative frequency
4. No spaces between groups.
II. DESCRIBING DISTRIBUTIONS
A. Shape
1. Modes – how many peaks or bumps
there are
i.
ii.
iii.
iv.
Uniform – no peaks
Unimodal – one peak
Bimodal – two peaks
Multimodal- more than two peaks.
2. Symmetry
i. Symmetric – equal on both sides of the center
ii. Skewed R – tail on R side
iii. Skewed L – tail on L side
II. DESCRIBING DISTRIBUTIONS
B. Center – the place on the graph that would
divide the area of the two halves equally.
i.
ii.
Mean – used when shape is symmetric or uniform
Median – used when shape is skewed.
C. Spread – how far away the data is from the
center (variability). Calculate the Range.
D. Outliers and Gaps – observation that differ from
the pattern.