Political Science 138:
Quantitative Reasoning in Political Science

HW8: The Boxplot  -- In class

1. Download this excel file
2. Prepare a boxplot that looks like the image.
3. Save the file, create a link.

HW9: The Boxplot  -- For your Term paper (if possible, otherwise a time series or scatter chart)

Read: Good Charts: Boxplots

other examples: Table 4.2

Creating a boxplot:

• First, construct an American-state or cross country dataset with one variable measured at different points in time, or for different groups of states or countries.
• Create a boxplot following the instruction below.

Collect data on you profile topic or use one of these data sets:

• State presidential election data
• State Homeownership rates
• State STD rates
• State 8th grade Math and Reading scores
• state poverty rates

About Boxplots:

Boxplots, also known as box-and-whisker plots, are a convenient method of displaying the distribution of an interval-level variables. 

Figure xx: Components of a boxplot

The simple boxplot, as shown in figure xx, displays the four quartiles of the data, with the "box" comprising the two middle quartiles, separated by the median.  The upper and lower quartiles are represented by the single lines extending from the box.

A single boxplot box (as in figure xx) rarely reveals much about the data; in most instances stem-and-leaf or histogram charts offer a more detailed graphic representation of the data distribution.  The real advantages of the boxplot graphic comes through, however, in charts that use several boxplot boxes to compare the distribution of a variable across groups or over time.

An especially useful elaboration of the boxplot graph involves plotting an individual case over the boxplot to compare a single cases to the overall  distribution. 

(click to expand thumbnail)

Figure 1: Time series boxplots, with individual cases  (data)

Thus figure 1 displays the percentage Democratic vote for the 50 states over the past seven presidential elections.  We can see that the Democratic vote in Nevada has moved steadily higher relative to the other states while Georgia has become an increasingly Republican state. 

One can easily imagine applying  the same plotting strategy in a variety of other settings, for example, comparing one school district's test scores to the distribution of test scores across other school districts.   Other examples.

To do the box charts, you will need the box charter add-in.  In the classroom, choose:

Tools | add-ins | box charter utility

At home, you must install the add-in on your computer.  Follow the instructions here:

The utility can be downloaded from here, which links to a zip file. The zip file contains two Excel files, the BoxCharter.xla add-in which does the work, and the Sample Box Chart Data.xls workbook, which has some sample data ranges to illustrate the operation of the utility.

Examples of boxplots:

data source:
spreadsheet

data source