Here’s the super short version of this data tip: when designing data viz, don’t mix complete and incomplete data. Below is the somewhat-longer-but-still-60-seconds version of this data tip.
The graphs below were on the Chicago Public Schools (CPS) website and show the number of confirmed COVID-19 cases associated with CPS buildings. I’ve added the pink lines and captions to direct your attention to the last two data points. The first image shows what the graph looked like on Monday, January 18th, and the second image shows what the graph looked like 5 days later on Saturday, January 23rd.
If you went to this website on January 18th and looked that this graph, you might very well have concluded that cases had recently plummeted. But you’d be wrong. To understand what was really going on, you’d have to notice that the last data point (1/23) was in the future and put that information together with a caption saying “Case counts are updated Monday through Friday (excluding holidays) after impacted individuals are notified.” So the graph shows complete data for past weeks but incomplete data for the current week. The last data point shows week-to-date data.
Source: Chicago Public Schools
Unless your aim is to confuse or deceive, why present data in this way? Instead, when you have complete data for various time periods or groups and incomplete data for other time periods or groups, consider the following:
If you are updating data on a daily basis, then show day intervals (rather than week intervals as in the graphs above) on the X-axis.
Create a separate chart showing a running total for the incomplete time period or group and place it alongside the graph showing complete data.
If neither of the solutions above work for you, at least color the dots, lines, or other marks representing the incomplete data in a color different from the complete data to alert the viewer to the difference and include a color legend to explain the difference.
Thanks to Carol White of CBWhite marketing research and strategy consulting for pointing out this graph to me! To see past data tips, click HERE.
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