Common Mistakes in Data Presentation - InformationWeek

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Common Mistakes in Data Presentation

When it comes to data presentation, graphical glitz can confuse rather than clarify what's being communicated. Our practical series delves into common errors and how to correct them.

Tables work best when the data presentation:

  • Is used to look up individual values
  • Is used to compare individual values
  • Requires precise values
  • Values involve multiple units of measure.

Graphs work best when the data presentation:

  • Is used to communicate a message that is contained in the shape of the data
  • Is used to reveal relationships among many values.

Use the Right Graph Type

Different types of graphs are designed to communicate different types of messages. You can't force a scatter plot to effectively communicate revenue for the last 12 months. You shouldn't force every kind of data into a bar graph just because that's the only kind you've learned to make. You aren't doing anyone any favors by forcing year-to-date sales by region into a radar graph just because it looks cool. Software vendors compete to outdo the competition by providing a cornucopia of graph types; time that would be better used helping their customers to understand and effectively use a smaller, more practical set of graphs.

Figure 1 shows an example on the top taken from Visual Mining's Web site of a graph that's inappropriate for the message, compared to one that I made on the bottom to illustrate a more appropriate choice.



FIGURE 1 Poor graph choice on the top vs. an effective choice on the bottom.

This example is somewhat extreme, but not any more absurd than countless graphs that I've seen in actual use. A more common example is the use of lines to encode data in a graph when bars would be more appropriate. The graph on the top in Figure 2 uses a line to encode the expenses of seven departments. Why is it more appropriate to use bars to encode this data, as I've done in the graph on the bottom?



FIGURE 2 The graph on the top illustrates an inappropriate use of lines to encode data, which is better encoded using bars, shown on the bottom.

Look at the slope of the line: As it moves from data point to data point in the graph, it suggests change between different instances of the same measure. The difference between each department's expenses is meaningful, but the movement from one department's expenses to the next doesn't represent a change. Bars would be a more appropriate choice to emphasize the independent nature of each department's expenses, as shown on the bottom. In contrast, it's very appropriate to use a line to encode the flow of values across time, such as consecutive months of a year. The movement from one value to the next in this case does represent change, so the slope of the line is meaningful: the steeper the slope, the more dramatic the change. Even when graphing time-series data, however, if you want your message to emphasize the comparison of values within each time period (for example, the comparison of revenue and operating expenses in each month in Figure 1), rather than how those values change through time — bars highlight this information more effectively.

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