Introductory Statistics 2e

2.1Stem-and-Leaf Graphs (Stemplots), Line Graphs, and Bar Graphs

Introductory Statistics 2e2.1 Stem-and-Leaf Graphs (Stemplots), Line Graphs, and Bar Graphs

One simple graph, the stem-and-leaf graph or stemplot, comes from the field of exploratory data analysis. It is a good choice when the data sets are small. To create the plot, divide each observation of data into a stem and a leaf. The leaf consists of a final significant digit. For example, 23 has stem two and leaf three. The number 432 has stem 43 and leaf two. Likewise, the number 5,432 has stem 543 and leaf two. The decimal 9.3 has stem nine and leaf three. Write the stems in a vertical line from smallest to largest. Draw a vertical line to the right of the stems. Then write the leaves in increasing order next to their corresponding stem.

Example 2.1

For Professor Dean's spring pre-calculus class, scores for the first exam were as follows (smallest to largest):
33; 42; 49; 49; 53; 55; 55; 61; 63; 67; 68; 68; 69; 69; 72; 73; 74; 78; 80; 83; 88; 88; 88; 90; 92; 94; 94; 94; 94; 96; 100

Stem Leaf
33
42 9 9
53 5 5
61 3 7 8 8 9 9
72 3 4 8
80 3 8 8 8
90 2 4 4 4 4 6
100
Table 2.1 Stem-and-Leaf Graph

The stemplot shows that most scores fell in the 60s, 70s, 80s, and 90s. Eight out of the 31 scores or approximately 26% $( 8 31 ) ( 8 31 )$ were in the 90s or 100, a fairly high number of As.

Try It 2.1

For the Park City basketball team, scores for the last 30 games were as follows (smallest to largest):
32; 32; 33; 34; 38; 40; 42; 42; 43; 44; 46; 47; 47; 48; 48; 48; 49; 50; 50; 51; 52; 52; 52; 53; 54; 56; 57; 57; 60; 61
Construct a stem plot for the data.

The stemplot is a quick way to graph data and gives an exact picture of the data. You want to look for an overall pattern and any outliers. An outlier is an observation of data that does not fit the rest of the data. It is sometimes called an extreme value. When you graph an outlier, it will appear not to fit the pattern of the graph. Some outliers are due to mistakes (for example, writing down 50 instead of 500) while others may indicate that something unusual is happening. It takes some background information to explain outliers, so we will cover them in more detail later.

Example 2.2

The data are the distances (in kilometers) from a home to local supermarkets. Create a stemplot using the data:
1.1; 1.5; 2.3; 2.5; 2.7; 3.2; 3.3; 3.3; 3.5; 3.8; 4.0; 4.2; 4.5; 4.5; 4.7; 4.8; 5.5; 5.6; 6.5; 6.7; 12.3

Problem

Do the data seem to have any concentration of values?

NOTE

The leaves are to the right of the decimal.

Try It 2.2

The following data show the distances (in miles) from the homes of off-campus statistics students to the college. Create a stem plot using the data and identify any outliers:

0.5; 0.7; 1.1; 1.2; 1.2; 1.3; 1.3; 1.5; 1.5; 1.7; 1.7; 1.8; 1.9; 2.0; 2.2; 2.5; 2.6; 2.8; 2.8; 2.8; 3.5; 3.8; 4.4; 4.8; 4.9; 5.2; 5.5; 5.7; 5.8; 8.0

Example 2.3

Problem

A side-by-side stem-and-leaf plot allows a comparison of the two data sets in two columns. In a side-by-side stem-and-leaf plot, two sets of leaves share the same stem. The leaves are to the left and the right of the stems. Table 2.3 and Table 2.4 show the ages of presidents at their inauguration and at their death. Construct a side-by-side stem-and-leaf plot using this data.

PresidentAgePresidentAgePresidentAge
Washington57Lincoln52Hoover54
Jefferson57Grant46Truman60
Monroe58Garfield49Kennedy43
Jackson61Cleveland47Nixon56
Van Buren54B. Harrison55Ford61
W. H. Harrison68Cleveland55Carter52
Tyler51McKinley54Reagan69
Polk49T. Roosevelt42G.H.W. Bush64
Taylor64Taft51Clinton47
Fillmore50Wilson56G. W. Bush54
Pierce48Harding55Obama47
Buchanan65Coolidge51
Table 2.3 Presidential Ages at Inauguration
PresidentAgePresidentAgePresidentAge
Washington67Lincoln56Hoover90
Jefferson83Grant63Truman88
Monroe73Garfield49Kennedy46
Jackson78Cleveland71Nixon81
Van Buren79B. Harrison67Ford93
W. H. Harrison68Cleveland71Reagan93
Tyler71McKinley58
Polk53T. Roosevelt60
Taylor65Taft72
Fillmore74Wilson67
Pierce64Harding57
Buchanan77Coolidge60
Table 2.4 Presidential Age at Death

Try It 2.3

The table shows the number of wins and losses the Atlanta Hawks have had in 42 seasons. Create a side-by-side stemand-leaf plot of these wins and losses.

Losses Wins Season Losses Wins Season
34 48 1 41 41 22
34 48 2 39 43 23
46 36 3 44 38 24
46 36 4 39 43 25
36 46 5 25 57 26
47 35 6 40 42 27
51 31 7 36 46 28
53 29 8 26 56 29
51 31 9 32 50 30
41 41 10 19 31 31
36 46 11 54 28 32
32 50 12 57 25 33
51 31 13 49 33 34
40 42 14 47 35 35
39 43 15 54 28 36
42 40 16 69 13 37
48 34 17 56 26 38
32 50 18 52 30 39
25 57 19 45 37 40
32 50 20 35 47 41
30 52 21 29 53 42
Table 2.6

Another type of graph that is useful for specific data values is a line graph. In the particular line graph shown in Example 2.4, the x-axis (horizontal axis) consists of data values and the y-axis (vertical axis) consists of frequency points. The frequency points are connected using line segments.

Example 2.4

In a survey, 40 parents were asked how many times per week a teenager must be reminded to do their chores. The results are shown in Table 2.7 and in Figure 2.2.

Number of times teenager is reminded Frequency
02
15
28
314
47
54
Table 2.7
Figure 2.2

Try It 2.4

In a survey, 40 people were asked how many times per year they had their car in the shop for repairs. The results are shown in Table 2.8. Construct a line graph.

Number of times in shopFrequency
07
110
214
39
Table 2.8

Bar graphs consist of bars that are separated from each other. The bars can be rectangles or they can be rectangular boxes (used in three-dimensional plots), and they can be vertical or horizontal. The bar graph shown in Example 2.5 has age groups represented on the x-axis and proportions on the y-axis.

Example 2.5

Problem

The percentage of U.S.-based TikTok users by age is shown in Table 2.9. Construct a bar graph using this data.

Age groups Proportion (%) of TikTok users
10–19 32.5%
20–29 29.5%
30–39 16.4%
40–49 13.9%
50+ 7.1%
Table 2.9

Try It 2.5

The population in Park City is made up of children, working-age adults, and retirees. Table 2.10 shows the three age groups, the number of people in the town from each age group, and the proportion (%) of people in each age group. Construct a bar graph showing the proportions.

Age groupsNumber of peopleProportion of population
Children 67,059 19%
Retirees 131,662 38%
Table 2.10

Example 2.6

Problem

The columns in Table 2.11 show the projected data for the year 2030 for the number and percentages of high school graduates by geographic region in the United States. Create a bar graph for this data with the geographic region (qualitative data) on the x-axis and the percentage of high school data (quantitative data) on the y-axis.

Northeast 517,720 16.1%
Midwest 695,170 21.6%
South 1,253,540 39.0%
West 749,400 23.3%
Table 2.11

Try It 2.6

Park city is broken down into six voting districts. The table shows the percent of the total registered voter population that lives in each district as well as the percent total of the entire population that lives in each district. Construct a bar graph that shows the registered voter population by district.

DistrictRegistered voter populationOverall city population
115.5%19.4%
212.2%15.6%
39.8%9.0%
417.4%18.5%
522.8%20.7%
622.3%16.8%
Table 2.12