Pie Charts

Pie Charts

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