- Our phones monitor our time and internet usage and online surveys discern our opinions and likes.
- To help the average person understand big data and numbers, we often use visual summaries, such as pie charts.
- But while non-numerate folk will avoid numbers, most numerate folk will avoid pie charts.
What is a pie chart?
- An example pie chart below shows Australia’s two-party preferred vote before the last election, with Labor on 55% and the the Coalition on 45%.
- The two near semi-circles show the relatively tight race – this is a useful example of a pie chart.
What’s wrong with pie charts?
Once we have more than two categories, pie charts can easily misrepresent percentages and become hard to read. The three charts below are a good example – it is very hard to work out which of the five areas is the largest. The pie chart’s circularity means the areas lack a common reference point.
Pie charts also do badly when there are lots of categories. For example, this chart from a study on data sources used for COVID data visualisation shows hundreds of categories in one pie.
The tiny slices, lack of clear labelling and the kaleidoscope of colours make interpretation difficult for anyone. It’s even harder for a colour blind person. For example, this is a simulation of what the above chart would look like to a person with deuteranomaly or reduced sensitivity to green light. This is the most common type of colour blindness, affecting roughly 4.6% of the population.
It can get even worse if we take pie charts and make them three-dimensional. This can lead to egregious misrepresentations of data. Below, the yellow, red and green areas are all the same size (one-third), but appear to be different based on the angle and which slice is placed at the bottom of the pie.
So why are pie charts everywhere?
- Despite the well known problems with pie charts, they are everywhere.
- While statisticians have criticised them for decades, it’s hard to argue with this logic: “if pie charts are so bad, why are there so many of them?” Possibly they are popular because they are popular, which is a circular argument that suits a pie chart.
What’s a good alternative to pie charts?
There’s a simple fix that can effectively summarise big data in a small space and still allow creative colour schemes. It’s the humble bar chart. Remember the brain-aching pie chart example above with the five categories? Here’s the same example using bars – we can now instantly see which category is the largest.
- Linear bars are easier on the eye than the non-linear segments of a pie chart.
- But beware the temptation to make a humble bar chart look more interesting by adding a 3D effect.
Is it ever okay to use a pie chart?
- Pie charts can be okay when there are just a few categories and the percentages are dissimilar, for example with one large and one small category.
- Overall, it is best to use pie charts sparingly, especially when there is a more “digestible” alternative – the bar chart.
Adrian Barnett is a member of the Statistical Society of Australia. Victor Oguoma is a member of the Statistical Society of Australia.
