2.1.1.2 - Visual Representations

2.1.1.2 - Visual Representations

Frequency tables, pie charts, and bar charts are the most appropriate graphical displays for categorical variables. Below are a frequency table, a pie chart, and a bar graph for data concerning Penn State’s undergraduate enrollments by campus in Fall 2017.

Note that in the bar chart, the bars are separated by a space. The spaces between the bars signify that this is a categorical variable. On the following pages you will learn how to make these graphs using Minitab Express.

Frequency Table

A table containing the counts of how often each category occurs.

Tally
Campus Count Percent
University Park 40835 48.5%
Commonwealth Campuses 29388 34.9%
PA College of Technology 5465 6.5%
World Campus 8513 10.1%
Total 84201 100.0%

Penn State Fall 2017 Undergraduate Enrollments

Pie chart

Graphical representation for categorical data in which a circle is partitioned into “slices” on the basis of the proportions of each category.

Pie Chart of Campus
Category
  •  University Park (48.5%)
  •  Commonwealth Campuses (34.9%)
  •  PA College of Technology (6.5%)
  •  World Campus (10.1%)
Penn State Fall 2017 Undergraduate Enrollments
Bar chart

Graphical representation for categorical data in which vertical (or sometimes horizontal) bars are used to depict the number of experimental units in each category; bars are separated by space.

Minitab Express Bar Chart for Fall 2017 Penn State Undergraduate Enrollments

Penn State Fall 2017 Undergraduate Enrollments

Tips

Pie charts tend to work best when there are only a few categories. If a variable has many categories, a pie chart may be more difficult to read. In those cases, a frequency table or bar chart may be more appropriate.

When selecting a visual display for your data you should first determine how many variables you are going to display and whether they are categorical or quantitative. Then, you should think about what you are trying to communicate. Each visual display has its own strengths and weaknesses. When first starting out, you may need to make a few different types of displays to determine which best communicates your data.


2.1.1.2.1 - Minitab Express: Frequency Tables

2.1.1.2.1 - Minitab Express: Frequency Tables

The following data set (from College Board) contain the mean SAT scores for each of the 50 states and Washington, DC, as well the participation rates and geographic region of each state. To get an idea of the pattern of variation of a categorical variable such as region, we can display the information with a frequency table, pie chart, or bar graph.

MinitabExpress  – Frequency Table

To create a frequency table in Minitab Express:

  1. Open the data set:
  2. On a PC: In the menu bar select STATISTICS > Describe > Tally
    On a Mac: In the menu bar select Statistics > Summary Statistics > Tally
  3. Double click the variable Region in the box on the left to insert the variable into the Variable box
  4. Under Statistics, check Counts and Percents
  5. Click OK

This should result in the following frequency table:

Tally
Region Count Percent
ENC 5 9.8039%
ESC 4 7.8431%
MA 3 5.8824%
MTN 8 15.6863%
NE 6 11.7647%
PAC 5 9.8039%
SA 9 17.6471%
WNC 7 13.7255%
WSC 4 7.8431%
N= 51  
Video Walkthrough

Select your operating system below to see a step-by-step guide for this example.


2.1.1.2.2 - Minitab Express: Pie Charts

2.1.1.2.2 - Minitab Express: Pie Charts

The following data set (from College Board) contain the mean SAT scores for each of the 50 states and Washington, DC, as well the participation rates and geographic region of each state. 

MinitabExpress  – Pie Chart (Raw Data)

To create a pie chart in Minitab Express:

  1. Open the data set:
  2. On a PC or Mac: Select Graphs > Pie Chart
  3. Select Counts of Unique Values
  4. Double click the variable Region in the box on the left to insert the variable into the Categorical variable box
  5. Click OK

This should result in the pie chart below:

Pie Charts created using Minitab Express
Video Walkthrough

Select your operating system below to see a step-by-step guide for this example.

Summarized Data

In the examples above raw data were used. In other words, the dataset contained one row for each case. It is also possible to use Minitab Express to construct a pie chart given summarized data, for example, if you had your counts in a frequency table. If this were the case, in step 3 you would select Summarized Data and enter the names of the categories in the Category names box and the frequency counts in the Summary values box.


2.1.1.2.3 - Minitab Express: Bar Charts

2.1.1.2.3 - Minitab Express: Bar Charts

The following data set (from College Board) contain the mean SAT scores for each of the 50 states and Washington, DC, as well the participation rates and geographic region of each state. 

MinitabExpress  – Bar Chart (Raw Data)

To create a bar graph in Minitab Express:

  1. Open the data set 
  2. On a PC or Mac: Select Graphs > Bar Chart
  3. Use the default from the drop down Bars represent of Counts of unique values in a categorical variable
  4. Select Simple
  5. Double click the variable Region in the box on the left to insert the variable into the Categorical variable box
  6. Click OK

This should result in the bar graph below:

Chart of Region

Video Walkthrough

Select your operating system below to see a step-by-step guide for this example.

Summarized Data

In the examples above raw data were used. In other words, the Minitab Express file consisted of one row for each case. We can also use Minitab Express to construct a bar chart with summarized data, for example, if you had data in a frequency table. To do this, in the third step shown above you will change the dropdown of Bars represent to Summarized values for each category in a table. You will still select Simple. The Summary variable will be the numerical values and the Categorical variable will be the names of the categories. 


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