Friday 17 April 2020

How Many Segments? And other stories

Following on from my recent presentation at the first ever Qlik Virtual Meetup Scotland (link opens the meeting recording), there were a few questions arising. I thought it would be useful to answer them in a blog post.

I like a pie chart but how many segments before it becomes difficult to judge a value?
Would you not just group smaller segments into OTHER?
Pies / Bars
I think the long tail on a bar chart, with the Sense minichart, is very obvious - where lots of tiny pie wedges do not

Let's start with the "how many segments" question and hopefully the thread will move through the answering of the others. First, consider this typical pie chart, produced in Qlik Sense:

A typical pie chart with many segments

As with the majority of visualization tools, Qlik Sense has defaulted to the sensible option of sorting the segments in size order. This immediately allows us to see that the USA is bigger than Germany which is bigger than Austria. I can roughly estimate the values of each (as shown in my research) and I can quickly tell that the three markets make up just under half of the whole.

The very simple answer to the question of "how many segments" is, how many makes sense to meet the business requirements.

On the question of grouping smaller segments, I can modify the original chart as below, and still answer those same business questions:

Pie chart showing the top 3 segments versus all others

In this situation, I can still see answer that business question and have reduced the number of segments on display. I would argue that I can answer that business question equally well with either chart, although the first may actually deliver me additional insights, and that is the critical thing about either chart - that they answer that business question.

What can sometimes be a problem, however, is that with interactive tools such as Qlik Sense the user could drill down to just those 3 countries:

Pie chart after user has drilled to 3 countries 

Now the user is no longer able to see the part-to-whole of these countries' market share. Instead, they are looking at the part-to-whole of just these countries. This may be OK! It depends on the business question that the user wants to answer. If it is a problem, you can use Set Analysis in Qlik Sense to do something about it - similar to one of my previous pie posts.

Let's consider some alternatives (alt-pie charts!), starting with the simple bar chart.

Typical bar chart showing a measure versus a categorical value

Again, we typically order these charts by value, so we can still quickly see that the US is larger than Germany and both are larger than Austria. If our purpose here is to compare one country versus others, then this is the perfect chart to use. Even given that, it is not really so easy to compare the US vs. Poland or even Belgium, but interactivity can help with this. It is, however, a quite a lot more difficult to see how much of the total market is made up by the top 3 countries. That would be especially more difficult if there were a longer tail of smaller values that you might have to scroll. That is why I prefer the pie chart if the business question is a part-to-whole one. We can, of course, clump the other countries into "Others" in the bar chart, but it is still not easy to see the part-to-whole:

Bar chart with "Others" bar

The recommended choice for part-to-whole coming from anti-pie advocates, is the horizontal bar:

Examples of horizontal bars as an alternative to pie charts

As my research has shown, the horizontal bar does not always work as well as a pie chart. It is a valid option, and one that you could consider, but it should definitely not be the default.

To summarize, if you are asking the question "how many segments", then you may be asking the wrong question. Always remember Redmond's Rules:

  • Use the right visual encodings (and pie charts are a valid choice!)
  • Add labels and annotations to provide context to the user
  • SFW! Make sure that you are answering the business question

The last rule can be hard, because sometimes you don't actually know the questions that the users want to answer! In those circumstances, following a Design Thinking methodology will probably get you where you want to be.

As well as holding a Master's Degree in Data Analytics, Stephen Redmond is a practicing Data Professional of over 20 years experience. He is author of Mastering QlikView, QlikView Server and Publisher and the QlikView for Developer's Cookbook

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