Descriptive Statistics
StemandLeaf Graphs (Stemplots), Line Graphs, and Bar Graphs
One simple graph, the stemandleaf 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.
For Susan Dean’s spring precalculus 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% were in the 90s or 100, a fairly high number of As.
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.
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
Do the data seem to have any concentration of values?
The leaves are to the right of the decimal.
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 
The following data show the distances (in miles) from the homes of offcampus 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
A sidebyside stemandleaf plot allows a comparison of the two data sets in two columns. In a sidebyside stemandleaf plot, two sets of leaves share the same stem. The leaves are to the left and the right of the stems. (Figure) and (Figure) show the ages of presidents at their inauguration and at their death. Construct a sidebyside stemandleaf plot using this data.
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 0 1 1 1 4 7 8 8 9  
8  0 1 3 5 8  
9  0 0 3 3 
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 
The table shows the number of wins and losses the Atlanta Hawks have had in 42 seasons. Create a sidebyside stemandleaf plot of these wins and losses.
Losses  Wins  Year  Losses  Wins  Year 

34  48  1968–1969  41  41  1989–1990 
34  48  1969–1970  39  43  1990–1991 
46  36  1970–1971  44  38  1991–1992 
46  36  1971–1972  39  43  1992–1993 
36  46  1972–1973  25  57  1993–1994 
47  35  1973–1974  40  42  1994–1995 
51  31  1974–1975  36  46  1995–1996 
53  29  1975–1976  26  56  1996–1997 
51  31  1976–1977  32  50  1997–1998 
41  41  1977–1978  19  31  1998–1999 
36  46  1978–1979  54  28  1999–2000 
32  50  1979–1980  57  25  2000–2001 
51  31  1980–1981  49  33  2001–2002 
40  42  1981–1982  47  35  2002–2003 
39  43  1982–1983  54  28  2003–2004 
42  40  1983–1984  69  13  2004–2005 
48  34  1984–1985  56  26  2005–2006 
32  50  1985–1986  52  30  2006–2007 
25  57  1986–1987  45  37  2007–2008 
32  50  1987–1988  35  47  2008–2009 
30  52  1988–1989  29  53  2009–2010 
Another type of graph that is useful for specific data values is a line graph. In the particular line graph shown in (Figure), the xaxis (horizontal axis) consists of data values and the yaxis (vertical axis) consists of frequency points. The frequency points are connected using line segments.
In a survey, 40 mothers were asked how many times per week a teenager must be reminded to do his or her chores. The results are shown in (Figure) and in (Figure).
Number of times teenager is reminded  Frequency 

0  2 
1  5 
2  8 
3  14 
4  7 
5  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 (Figure). 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 threedimensional plots), and they can be vertical or horizontal. The bar graph shown in (Figure) has age groups represented on the xaxis and proportions on the yaxis.
By the end of 2011, Facebook had over 146 million users in the United States. (Figure) shows three age groups, the number of users in each age group, and the proportion (%) of users in each age group. Construct a bar graph using this data.
Age groups  Number of Facebook users  Proportion (%) of Facebook users 

13–25  65,082,280  45% 
26–44  53,300,200  36% 
45–64  27,885,100  19% 
The population in Park City is made up of children, workingage adults, and retirees. (Figure) 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% 
Workingage adults  152,198  43% 
Retirees  131,662  38% 
The columns in (Figure) contain: the race or ethnicity of students in U.S. Public Schools for the class of 2011, percentages for the Advanced Placement examine population for that class, and percentages for the overall student population. Create a bar graph with the student race or ethnicity (qualitative data) on the xaxis, and the Advanced Placement examinee population percentages on the yaxis.
Race/Ethnicity  AP Examinee Population  Overall Student Population 

1 = Asian, Asian American or Pacific Islander  10.3%  5.7% 
2 = Black or African American  9.0%  14.7% 
3 = Hispanic or Latino  17.0%  17.6% 
4 = American Indian or Alaska Native  0.6%  1.1% 
5 = White  57.1%  59.2% 
6 = Not reported/other  6.0%  1.7% 
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% 
References
Burbary, Ken. Facebook Demographics Revisited – 2001 Statistics, 2011. Available online at http://www.kenburbary.com/2011/03/facebookdemographicsrevisited2011statistics2/ (accessed August 21, 2013).
“9th Annual AP Report to the Nation.” CollegeBoard, 2013. Available online at http://apreport.collegeboard.org/goalsandfindings/promotingequity (accessed September 13, 2013).
“Overweight and Obesity: Adult Obesity Facts.” Centers for Disease Control and Prevention. Available online at http://www.cdc.gov/obesity/data/adult.html (accessed September 13, 2013).
Chapter Review
A stemandleaf plot is a way to plot data and look at the distribution. In a stemandleaf plot, all data values within a class are visible. The advantage in a stemandleaf plot is that all values are listed, unlike a histogram, which gives classes of data values. A line graph is often used to represent a set of data values in which a quantity varies with time. These graphs are useful for finding trends. That is, finding a general pattern in data sets including temperature, sales, employment, company profit or cost over a period of time. A bar graph is a chart that uses either horizontal or vertical bars to show comparisons among categories. One axis of the chart shows the specific categories being compared, and the other axis represents a discrete value. Some bar graphs present bars clustered in groups of more than one (grouped bar graphs), and others show the bars divided into subparts to show cumulative effect (stacked bar graphs). Bar graphs are especially useful when categorical data is being used.
For each of the following data sets, create a stem plot and identify any outliers.The miles per gallon rating for 30 cars are shown below (lowest to highest).
19, 19, 19, 20, 21, 21, 25, 25, 25, 26, 26, 28, 29, 31, 31, 32, 32, 33, 34, 35, 36, 37, 37, 38, 38, 38, 38, 41, 43, 43
Stem  Leaf 

1  9 9 9 
2  0 1 1 5 5 5 6 6 8 9 
3  1 1 2 2 3 4 5 6 7 7 8 8 8 8 
4  1 3 3 
The height in feet of 25 trees is shown below (lowest to highest).
25, 27, 33, 34, 34, 34, 35, 37, 37, 38, 39, 39, 39, 40, 41, 45, 46, 47, 49, 50, 50, 53, 53, 54, 54
The data are the prices of different laptops at an electronics store. Round each value to the nearest ten.
249, 249, 260, 265, 265, 280, 299, 299, 309, 319, 325, 326, 350, 350, 350, 365, 369, 389, 409, 459, 489, 559, 569, 570, 610
Stem  Leaf 

2  5 5 6 7 7 8 
3  0 0 1 2 3 3 5 5 5 7 7 9 
4  1 6 9 
5  6 7 7 
6  1 
The data are daily high temperatures in a town for one month.
61, 61, 62, 64, 66, 67, 67, 67, 68, 69, 70, 70, 70, 71, 71, 72, 74, 74, 74, 75, 75, 75, 76, 76, 77, 78, 78, 79, 79, 95
For the next three exercises, use the data to construct a line graph.
In a survey, 40 people were asked how many times they visited a store before making a major purchase. The results are shown in (Figure).
Number of times in store  Frequency 

1  4 
2  10 
3  16 
4  6 
5  4 
In a survey, several people were asked how many years it has been since they purchased a mattress. The results are shown in (Figure).
Years since last purchase  Frequency 

0  2 
1  8 
2  13 
3  22 
4  16 
5  9 
Several children were asked how many TV shows they watch each day. The results of the survey are shown in (Figure).
Number of TV Shows  Frequency 

0  12 
1  18 
2  36 
3  7 
4  2 
The students in Ms. Ramirez’s math class have birthdays in each of the four seasons. (Figure) shows the four seasons, the number of students who have birthdays in each season, and the percentage (%) of students in each group. Construct a bar graph showing the number of students.
Seasons  Number of students  Proportion of population 

Spring  8  24% 
Summer  9  26% 
Autumn  11  32% 
Winter  6  18% 
Using the data from Mrs. Ramirez’s math class supplied in (Figure), construct a bar graph showing the percentages.
David County has six high schools. Each school sent students to participate in a countywide science competition. (Figure) shows the percentage breakdown of competitors from each school, and the percentage of the entire student population of the county that goes to each school. Construct a bar graph that shows the population percentage of competitors from each school.
High School  Science competition population  Overall student population 

Alabaster  28.9%  8.6% 
Concordia  7.6%  23.2% 
Genoa  12.1%  15.0% 
Mocksville  18.5%  14.3% 
Tynneson  24.2%  10.1% 
West End  8.7%  28.8% 
Use the data from the David County science competition supplied in (Figure). Construct a bar graph that shows the countywide population percentage of students at each school.
Homework
Student grades on a chemistry exam were: 77, 78, 76, 81, 86, 51, 79, 82, 84, 99
 Construct a stemandleaf plot of the data.
 Are there any potential outliers? If so, which scores are they? Why do you consider them outliers?
(Figure) contains the 2010 obesity rates in U.S. states and Washington, DC.
State  Percent (%)  State  Percent (%)  State  Percent (%) 

Alabama  32.2  Kentucky  31.3  North Dakota  27.2 
Alaska  24.5  Louisiana  31.0  Ohio  29.2 
Arizona  24.3  Maine  26.8  Oklahoma  30.4 
Arkansas  30.1  Maryland  27.1  Oregon  26.8 
California  24.0  Massachusetts  23.0  Pennsylvania  28.6 
Colorado  21.0  Michigan  30.9  Rhode Island  25.5 
Connecticut  22.5  Minnesota  24.8  South Carolina  31.5 
Delaware  28.0  Mississippi  34.0  South Dakota  27.3 
Washington, DC  22.2  Missouri  30.5  Tennessee  30.8 
Florida  26.6  Montana  23.0  Texas  31.0 
Georgia  29.6  Nebraska  26.9  Utah  22.5 
Hawaii  22.7  Nevada  22.4  Vermont  23.2 
Idaho  26.5  New Hampshire  25.0  Virginia  26.0 
Illinois  28.2  New Jersey  23.8  Washington  25.5 
Indiana  29.6  New Mexico  25.1  West Virginia  32.5 
Iowa  28.4  New York  23.9  Wisconsin  26.3 
Kansas  29.4  North Carolina  27.8  Wyoming  25.1 
 Use a random number generator to randomly pick eight states. Construct a bar graph of the obesity rates of those eight states.
 Construct a bar graph for all the states beginning with the letter “A.”
 Construct a bar graph for all the states beginning with the letter “M.”
 Example solution for using the random number generator for the TI84+ to generate a simple random sample of 8 states. Instructions are as follows.
 Number the entries in the table 1–51 (Includes Washington, DC; Numbered vertically)
 Press MATH
 Arrow over to PRB
 Press 5:randInt(
 Enter 51,1,8)
Eight numbers are generated (use the right arrow key to scroll through the numbers). The numbers correspond to the numbered states (for this example: {47 21 9 23 51 13 25 4}. If any numbers are repeated, generate a different number by using 5:randInt(51,1)). Here, the states (and Washington DC) are {Arkansas, Washington DC, Idaho, Maryland, Michigan, Mississippi, Virginia, Wyoming}.
Corresponding percents are {30.1, 22.2, 26.5, 27.1, 30.9, 34.0, 26.0, 25.1}.
