Chapter 3 Graphical Methods for Describing Data
Double Bar Charts When to Use Categorical data How to construct • Constructed like bar charts, but with two (or more) groups being compared • MUST use relative frequencies on the vertical axis • MUST include a key to denote the different bars Why MUST we use relative frequencies?
Each year the Princeton Review conducts a survey of students applying to college and of parents of college applicants. In 2009, 12,715 high school students responded to the question “Ideally how far from home would you like the college you attend to be?” Also, 3007 parents of students applying to college responded to the question “how far from home would you like the college your child attends to be?” Data is displayed in the frequency table below. What should you do first? Create a comparative bar chart with these data.
Found by dividing the frequency by the total number of students Found by dividing the frequency by the total number of parents What does this graph show about the ideal distance college should be from home?
Segmented (or Stacked) Bar Charts When to Use Categorical data How to construct • MUST first calculate relative frequencies • Draw a bar representing 100% of the group • Divide the bar into segments corresponding to the relative frequencies of the categories
Remember the Princeton survey . . . Create a segmented bar graph with these data. First draw a bar that represents 100% of the students who answered the survey.
Relative frequency Less than 250 miles 250 to 500 miles 500 to 1000 miles More than 1000 miles Students Notice that this segmented bar chart displays the same relationship between the opinions of students and parents concerning the ideal distance that college is from home as the double bar chart does. First draw a bar that represents 100% of the students who answered the survey. Next, divide the bar into segments. Do the same thing for parents – don’t forget a key denoting each category Parents
Pie (Circle) Chart When to UseCategorical data How to construct • Draw a circle to represent the entire data set • Calculate the size of each “slice”: Relative frequency × 360° • Using a protractor, mark off each slice To describe – comment on which category had the largest proportion or smallest proportion
Typos on a résumé do not make a very good impression when applying for a job. Senior executives were asked how many typos in a résumé would make them not consider a job candidate. The resulting data are summarized in the table below. Create a pie chart for these data.
What does this pie chart tell us about the number of typos occurring in résumés before the applicant would not be considered for a job? First draw a circle to represent the entire data set. Next, calculate the size of the slice for “1 typo” .40×360º =144º Draw that slice. Repeat for each slice. Here is the completed pie chart created using Minitab.
What strikes you as the most distinctive difference among the distributions of exam scores in classes A, B, & C ?
1. Center • discuss where the middle of the data falls • three measures of central tendency • mean, median, & mode The mean and/or median is typically reported rather than the mode.
What strikes you as the most distinctive difference among the distributions of scores in classes D, E, & F?
2. Spread • discuss how spread out the data is • refers to the variability in the data • Measure of spread are • Range, standard deviation, IQR Remember, Range = maximum value – minimum value Standard deviation & IQR will be discussed in Chapter 4
What strikes you as the most distinctive difference among the distributions of exam scores in classes G, H, & I ?
3. Shape • refers to the overall shape of the distribution • symmetrical, uniform, skewed, or bimodal The following slides will discuss these shapes.
Symmetrical 1. Collect data by rolling two dice and recording the sum of the two dice. Repeat three times. 2. Plot your sums on the dotplot on the board. 3. What shape does this distribution have? • refers to data in which both sides are (more or less) the same when the graph is folded vertically down the middle • bell-shaped is a special type • has a center mound with two sloping tails
Uniform 1. Collect data by rolling a single die and recording the number rolled. Repeat five times. 2. Plot your numbers on the dotplot on the board. 3. What shape does this distribution have? • refers to data in which every class has equal or approximately equal frequency To help remember the name for this shape, picture soldier standing in straight lines. What are they wearing?
Skewed 1. Collect data finding the age of five coins in circulation (current year minus year of coin) and record 2. Plot the ages on the dotplot on the board. 3. What shape does this distribution have? Name a variable with a distribution that is negatively skewed. • refers to data in which one side (tail) is longer than the other side • the direction of skewness is on the side of the longer tail The directions are positively (or right) skewed or negatively (or left) skewed.
Bimodal (multi-modal) Suppose collect data on the time it takes to drive from San Luis Obispo, California to Monterey, California. Some people may take the inland route (approximately 2.5 hours) while others may take the coastal route (between 3.5 and 4 hours). What shape would this distribution have? • refers to the number of peaks in the shape of the distribution • Bimodal would have two peaks • Multi-modal would have more than two peaks Bimodal distributions can occur when the data set consist of observations from two different kinds of individuals or objects. What would a distribution be called if it had ONLY one peak? Unimodal
What strikes you as the most distinctive difference among the distributions of exam scores in class J ?
4. Unusual occurrences • Outlier - value that lies away from the rest of the data • Gaps • Clusters
5. In context • You must write your answer in reference to the context in the problem, using correct statistical vocabulary and using complete sentences!
Stem-and-Leaf Displays When to Use Univariate numerical data How to construct • Select one or more of the leading digits for the stem • List the possible stem values in a vertical column • Record the leaf for each observation beside each corresponding stem value • Indicate the units for stems and leaves in a key or legend To describe – comment on the center, spread, and shape of the distribution and if there are any unusual features Each number is split into two parts: Stem – consists of the first digit(s) Leaf - consists of the final digit(s) Can also create comparative stem-and-leaf displays Remember the data set collected in Chapter 1 – how many piercings do you have? Would a stem-and-leaf display be a good graph for this distribution? Why or why not? Use for small to moderate sized data sets. Doesn’t work well for large data sets. Be sure to list every stem from the smallest to the largest value If you have a long lists of leaves behind a few stems, you can split stems in order to spread out the distribution.
The following data are price per ounce for various brands of different brands of dandruff shampoo at a local grocery store. 0.32 0.21 0.29 0.54 0.17 0.28 0.36 0.23 Create a stem-and-leaf display with this data? What would an appropriate stem be? For the observation of “0.32”, write the 2 behind the “3” stem. List the stems vertically The median price per ounce for dandruff shampoo is $0.285, with a range of $0.37. The distribution is positively skewed with an outlier at $0.54. Continue recording each leaf with the corresponding stem Describe this distribution. 7 1 9 8 3 2 6 4
The Census Bureau projects the median age in 2030 for the 50 states and Washington D.C. A stem-and-leaf display is shown below. Notice that now you can see the shape of this distribution. We use L for lower leaf values (0-4) and H for higher leaf values (5-9). Notice that you really cannot see a distinctive shape for this distribution due to the long list of leaves We can split the stems in order to better see the shape of the distribution.
The median percentage of primary-school-aged children enrolled in school is larger for countries in Northern Africa than in Central Africa, but the ranges are the same. The distribution for countries in Northern Africa is strongly negatively skewed, but the distribution for countries in Central Africa is approximately symmetrical. The following is data on the percentage of primary-school-aged children who are enrolled in school for 19 countries in Northern Africa and for 23 countries in Central African. Northern Africa 54.6 34.3 48.9 77.8 59.6 88.5 97.4 92.5 83.9 98.8 91.6 97.8 96.1 92.2 94.9 98.6 86.6 96.9 88.9 Central Africa 58.3 34.6 35.5 45.4 38.6 63.8 53.9 61.9 69.9 43.0 85.0 63.4 58.4 61.9 40.9 73.9 34.8 74.4 97.4 61.0 66.7 79.6 Let’s truncate the leaves to the unit place. “4.6” becomes “4” Create a comparative stem-and-leaf display. Be sure to use comparative language when describing these distributions! What is an appropriate stem?
Histograms When to Use Univariate numerical data How to construct Discrete data • Draw a horizontal scale and mark it with the possible values for the variable • Draw a vertical scale and mark it with frequency or relative frequency • Above each possible value, draw a rectangle centered at that value with a height corresponding to its frequency or relative frequency To describe – comment on the center, spread, and shape of the distribution and if there are any unusual features Constructed differently for discrete versus continuous data For comparative histograms – use two separate graphs with the same scale on the horizontal axis
Queen honey bees mate shortly after they become adults. During a mating flight, the queen usually takes several partners, collecting sperm that she will store and use throughout the rest of her life. A study on honey bees provided the following data on the number of partners for 30 queen bees. 12 2 4 6 6 7 8 7 8 11 8 3 5 6 7 10 1 9 7 6 9 7 5 4 7 4 6 7 8 10 Create a histogram for the number of partners of the queen bees.
Draw a rectangle above each value with a height corresponding to the frequency. First draw a horizontal axis, scaled with the possible values of the variable of interest. Next draw a vertical axis, scaled with frequency or relative frequency. Suppose we use relative frequency instead of frequency on the vertical axis. What do you notice about the shapes of these two histograms?
Histograms When to Use Univariate numerical data How to construct Continuous data • Mark the boundaries of the class intervals on the horizontal axis • Draw a vertical scale and mark it with frequency or relative frequency • Draw a rectangle directly above each class interval with a height corresponding to its frequency or relative frequency To describe – comment on the center, spread, and shape of the distribution and if there are any unusual features This is the type of histogram that most students are familiar with.
Children Age 1 Children Age 3 The median number of hours spent watching TV per day was greater for the 1-year-olds than for the 3-year-olds. The distribution for the 3-year-olds was more strongly skewed right than the distribution for the 1-year-olds, but the two distributions had similar ranges. A study examined the length of hours spent watching TV per day for a sample of children age 1 and for a sample of children age 3. Below are comparative histograms. Notice the common scale on the horizontal axis Write a few sentences comparing the distributions.
Histograms with unequal intervals When to use - when you have a concentration of data in the middle with some extreme values How to construct - construct similar to histograms with continuous data, but with density on the vertical axis
Cumulative Relative Frequency Plot When to use - used to answer questions about percentiles. How to construct - Mark the boundaries of the intervals on the horizontal axis - Draw a vertical scale and mark it with relative frequency - Plot the point corresponding to the upper end of each interval with its cumulative relative frequency, including the beginning point - Connect the points. Percentiles are a value with a given percent of observations at or below that value.
Find the cumulative relative frequency for each interval The National Climatic Center has been collecting weather data for many years. The annual rainfall amounts for Albuquerque, New Mexico from 1950 to 2008 were used to create the frequency distribution below. 0.052 + 0.155 + 0.241 Continue this pattern to complete the table
To create a cumulative relative frequency plot, graph a point for the upper value of the interval and the cumulative relative frequency The National Climatic Center has been collecting weather data for many years. The annual rainfall amounts for Albuquerque, New Mexico from 1950 to 2008 were used to create the frequency distribution below. In the context of this problem, explain the meaning of this value. Why isn’t this value one (1)? Plot a point for each interval. Plot a starting point at (4,0). Connect the points. In the context of this problem, explain the meaning of this value.
Cumulative relative frequency Rainfall What proportion of years had rainfall amounts that were 9.5 inches or less? Approximately 0.55
Cumulative relative frequency Rainfall Approximately 30% of the years had annual rainfall less than what amount? Approximately 7.5 inches
Cumulative relative frequency Rainfall Which interval of rainfall amounts had a larger proportion of years – 9 to 10 inches or 10 to 11 inches? Explain The interval 10 to 11 inches, because its slope is steeper, indicating a larger proportion occurred.
Scatterplots When to Use Bivariate numerical data How to construct - Draw a horizontal scale and mark it with appropriate values of the independent variable - Draw a vertical scale and mark it appropriate values of the dependent variable - Plot each point corresponding to the observations To describe - comment the relationship between the variables Scatterplots are discussed in much greater depth in Chapter 5.
Time Series Plots When to Use - measurements collected over time at regular intervals How to construct - Draw a horizontal scale and mark it with appropriate values of time - Draw a vertical scale and mark it appropriate values of the observed variable - Plot each point corresponding to the observations and connect To describe - comment on any trends or patterns over time Can be considered bivariate data where the y-variable is the variable measured and the x-variableis time
The accompanying time-series plot of movie box office totals (in millions of dollars) over 18 weeks in the summer for 2001 and 2002 appeared in USA Today (September 3, 2002). Describe any trends or patterns that you see.