Pie charts are popular, they are used often and consume space that can be used for more information. What information does a pie chart give you? The ratio between several different numbers, nothing less and unfortunately nothing more.
Quality management in a contact centre is about gathering information, analyzing and taking action on it. We have to choose the right data and show them in a way that helps users to see the things they are interested in immediately.
You can express some information in many different ways. The meaning will still be the same. Choosing a right chart for a right use case may help users quickly see what is important. Talking about pie charts, you can express the same information in the following ways:
Of course, you can show this information in many other ways as well. The table shows exactly the same information as all the charts. Compared to the other charts it is quite difficult to see one piece of information that may be important for users – the ratio between the three values. Users have to read all the numbers and then calculate it. Even when done subconsciously it is still much slower to get the same information as you would get from the other charts.
It seems that the pie chart is perfect for the task. However it has one major disadvantage. It uses a lot of space and ink for too little information. You can, for example, use a stacked bar chart instead of a pie chart to include historic snapshots. By doing this you show users how the ratio and total amount have evolved over time. See the following example:
It is easy to imagine that you can fit more years into the same bar chart by making the bars thinner. Of course you can express history using pie charts. Even for just three years the pie chars do not look particularly great:
The bar chart may be used to show the history of the last twenty years without problems. You can try showing twenty years using pie charts yourself. You would notice that pie charts do not look well when they get small.
Also notice from the above example that it is easy to compare the total amounts in the bar chart. You can instantly see that 2009 is roughly 50% higher than 2008 and 2010 is slightly lower than 2009. This is not so obvious in the pie charts. The problem with comparing the amounts is also related to the fact that a two dimensional area expresses a one dimensional value.
The quote “The only worse design than a pie chart is several of them.” by Edward Tufte tells it all. His book The Visual Display of Quantitative Information is a great starting point for anyone interested in designing useful informative charts.
So should you avoid using pie charts completely? No. Just think carefully before using them. If you want to express just a ratio between few quantities a pie chart may be a way to go. Do you want to show your dominant market share to impress people in a press release? The pie chart does that and does it really well.