An important part of decision making is having a clear understanding of the information used to base decisions from. Charts can be valuable when a need to represent numerical data would benefit communicating information visually. Some of the most important aspects of a good chart are to select the right type of chart (or graph) that can best characterize the data, also, to keep the design simple in order for an audience to easily understand the information.
One of the most popular types of charts is the pie chart. The pie chart is used to visually represent the proportional value of individual parts to the whole. As the name describes, this is done by representing the numerical equivalence of each part as a piece of the whole pie, which in total equates to 100%. The Pennsylvania Department of Health (2001) says that pie charts are a good choice when a relatively small amount of parts, perhaps 3 to 7, need to be represented. With any more it becomes difficult to notice the differences in magnitude; thus, the pie chart loses its simplicity and impact. They can only be used when a total amount is known, one such example would be an election where the total of votes received by all candidates equals 100% of the votes. Or a budget where the total amount spending is divided in to categories such as labor, facilities costs, advertising, etc which always are a part of the total. However, according to McBride (2003), the pie chart could not be used to show a change in spending through out a period. A pie chart shows data at one instance, like a snapshot and cannot be used to show change in data over time (para. 4). With the advent of computers, 3D graphs have become somewhat popular, unfortunately a negative aspect is that they add complexity to the image and can distort visual proportional value. It is recommended to stick with flat “2D” charts (para. 6).
Pie charts these days are typically generated by computer software, as a result people forget that there is actually a little math involved. To create a pie chart Concordia University (2000) explains that you have to find the total value for the entire category being studied and calculate the percentage for each segment or part. Convert the percentage values for each segment into degrees relative to the 360 degrees in the circle. (For example, 12% X 360 degrees = 43 degrees). Then, Draw a circle and divide it into appropriately sized segments.
Sykes and Smith recommend that the sectors of the pie are ordered from largest to the smallest for easier interpretation of the data and they should be drawn in the counter-clockwise direction. (para. 7) For clarity a complete title of the data being represented is used, along with each slice clearly labeled to identify its characteristics, and the source of the data. Appropriate colors and or shading are also quite valuable in distinguishing the segments. Test the chart in its final format for readability. Whether it be displayed on a projector, paper or computer screen. With the assistance of a coworker, ensure that the audience will be able to clearly interpret the graph, making sure the chart is large and clear enough to read.
Based on what you have learned, what can you point out about the following chart that does not follow best practices?
Source: Where is earth’s water located? (2003)
2 pie charts were used in the single representation, this could be acceptable if the reader was given free time to study the chart, but the complexity of using one pie chart to further divide a portion of the other pie chart would be too confusing for many situations. Ideally it would be presented separately. Remember to keep it simple.
Adding to the confusion is that the same color was used on the 2 different charts, but represent 2 different things.
You may have noticed that the chart does not give the actual stats for the “Water usable by humans”. A chart is used to support statistics; and it is best to include the numbers in the chart.
The extreme 3D effect distorts the visual representation of the pie charts, which is the main reason for using a pie chart in the first place.
Now that you have had the chance to point out some things about a pie chart that can be wrong, what can you find in the next pie chart that was done right?
Source: Grandfather Economic Report series (2004)
Clear title describing the purpose of the chart.
Each section is clearly labeled with the statistic included.
Colors were used well to distinguish the pieces.
A reasonable # of pieces were used, not overloading the chart.
Overall this is an excellent example of what a pie chart should look like with the exception of one rule, the pieces were not arranged from largest to smallest in a clockwise manner. The sizes are mixed, making less clear the relative proportions of the pieces.
Pie charts can be an excellent tool for presenting information in a simple, visual and comprehensible manner. If used in the right situation and best practices are followed, the pie chart may be the tool that most influences a decision.
Pennsylvania Department of Health. (2001). Pie charts and pictographs. Retrieved January 24, 2005 from, http://www.health.state.pa.us/hpa/stats/ techassist/piechart.htm
Concordia University. (2000). Pie chart. Retrieved January 31, 2005 from, http://web2.concordia.ca/Quality/tools/21piechart.pdf
Sykes, A., ; Smith, B. (1999). Getting started – A simple pie chart. Retrieved January 31, 2005 from, http://www.causeway.co.uk/tutorial/rainpro/apl99/dyalog/start.htm
Hodges, M. (2004). Grandfather economic report series. Retrieved January 31, 2005 from, http://mwhodges.home.att.net/fed_budget.htm
Perlman, H. (2003). Where is earth’s water located? Retrieved January 31, 2005 from, http://ga.water.usgs.gov/edu/earthwherewater.html