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 |
---|---|

3 | 3 |

4 | 2 9 9 |

5 | 3 5 5 |

6 | 1 3 7 8 8 9 9 |

7 | 2 3 4 8 |

8 | 0 3 8 8 8 |

9 | 0 2 4 4 4 4 6 |

10 | 0 |

The stemplot shows that most scores fell in the 60s, 70s, 80s, and 90s. Eight out of the 31 scores or approximately 26% $\left(\frac{8}{31}\right)$ 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.

### Solution

The value 12.3 may be an outlier. Values appear to concentrate at three and four kilometers.

Stem | Leaf |
---|---|

1 | 1 5 |

2 | 3 5 7 |

3 | 2 3 3 5 8 |

4 | 0 2 5 5 7 8 |

5 | 5 6 |

6 | 5 7 |

7 | |

8 | |

9 | |

10 | |

11 | |

12 | 3 |

## 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.

President | Age | President | Age | President | Age |
---|---|---|---|---|---|

Washington | 57 | Lincoln | 52 | Hoover | 54 |

J. Adams | 61 | A. Johnson | 56 | F. Roosevelt | 51 |

Jefferson | 57 | Grant | 46 | Truman | 60 |

Madison | 57 | Hayes | 54 | Eisenhower | 62 |

Monroe | 58 | Garfield | 49 | Kennedy | 43 |

J. Q. Adams | 57 | Arthur | 51 | L. Johnson | 55 |

Jackson | 61 | Cleveland | 47 | Nixon | 56 |

Van Buren | 54 | B. Harrison | 55 | Ford | 61 |

W. H. Harrison | 68 | Cleveland | 55 | Carter | 52 |

Tyler | 51 | McKinley | 54 | Reagan | 69 |

Polk | 49 | T. Roosevelt | 42 | G.H.W. Bush | 64 |

Taylor | 64 | Taft | 51 | Clinton | 47 |

Fillmore | 50 | Wilson | 56 | G. W. Bush | 54 |

Pierce | 48 | Harding | 55 | Obama | 47 |

Buchanan | 65 | Coolidge | 51 |

President | Age | President | Age | President | Age |
---|---|---|---|---|---|

Washington | 67 | Lincoln | 56 | Hoover | 90 |

J. Adams | 90 | A. Johnson | 66 | F. Roosevelt | 63 |

Jefferson | 83 | Grant | 63 | Truman | 88 |

Madison | 85 | Hayes | 70 | Eisenhower | 78 |

Monroe | 73 | Garfield | 49 | Kennedy | 46 |

J. Q. Adams | 80 | Arthur | 56 | L. Johnson | 64 |

Jackson | 78 | Cleveland | 71 | Nixon | 81 |

Van Buren | 79 | B. Harrison | 67 | Ford | 93 |

W. H. Harrison | 68 | Cleveland | 71 | Reagan | 93 |

Tyler | 71 | McKinley | 58 | ||

Polk | 53 | T. Roosevelt | 60 | ||

Taylor | 65 | Taft | 72 | ||

Fillmore | 74 | Wilson | 67 | ||

Pierce | 64 | Harding | 57 | ||

Buchanan | 77 | Coolidge | 60 |

### Solution

Ages at Inauguration | Ages at Death | |
---|---|---|

9 9 8 7 7 7 6 3 2 | 4 | 6 9 |

8 7 7 7 7 6 6 6 5 5 5 5 4 4 4 4 4 2 2 1 1 1 1 1 0 | 5 | 3 6 6 7 7 8 |

9 8 5 4 4 2 1 1 1 0 | 6 | 0 0 3 3 4 4 5 6 7 7 7 8 |

7 | 0 1 1 1 2 3 4 7 8 8 9 | |

8 | 0 1 3 5 8 | |

9 | 0 0 3 3 |

## 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 |

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

**(vertical axis) consists of**

*y*-axis**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 |
---|---|

0 | 2 |

1 | 5 |

2 | 8 |

3 | 14 |

4 | 7 |

5 | 4 |

## 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 shop | Frequency |
---|---|

0 | 7 |

1 | 10 |

2 | 14 |

3 | 9 |

**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% |

### Solution

## 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 groups | Number of people | Proportion of population |
---|---|---|

Children | 67,059 | 19% |

Working-age adults | 152,198 | 43% |

Retirees | 131,662 | 38% |

## 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.

Region | Number of Graduates | Percentage of Graduates |
---|---|---|

Northeast | 517,720 | 16.1% |

Midwest | 695,170 | 21.6% |

South | 1,253,540 | 39.0% |

West | 749,400 | 23.3% |

### Solution

## 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.

District | Registered voter population | Overall city population |
---|---|---|

1 | 15.5% | 19.4% |

2 | 12.2% | 15.6% |

3 | 9.8% | 9.0% |

4 | 17.4% | 18.5% |

5 | 22.8% | 20.7% |

6 | 22.3% | 16.8% |