Why you need to pay special attention to the Y axes on Charts

Dec 28, 2017

In marketing, and especially media, we see a lot of charts.  Very often the Y axes are shown to benefit the point of the chart.  It’s always good practise to check the axes – it could be showing a very small variance (in number) but it might look big.

Understand bases, indexes and filters

I’m always baffled at how often data is misrepresented because of fundamental misunderstandings of simple concepts such as bases, indexes, and filters.

  • Bases: This is the number that everything is compared to.  Comparing the number of young Gauteng women who use cellphones to all South Africans will show you a story that they really use phones a lot but comparing that number to all Gauteng residents (men and women) won’t show much change.  If your market is Gauteng already (or high-income consumers) then it probably isn’t a good idea to compare to All South Africans – it gives a distorted picture.
  • Indexes:  Indexes (or Indices) is quite simple a calculation of the figure over the base.  In the above example, the Index is (the number of Gauteng Women using cellphones) divided by (All South Africans) or, as it’s put later, might be better shown as divided by (All Gauteng residents)
  • Filters: these are the group you are looking at – in the above example – Gauteng Women.


The y axes can change the outcome

If you’ve ever presented a chart that tells a story which your audience would prefer wasn’t true, you’ll undoubtedly have encountered that one person that questions the research, the sample, the base, or the use of the axes.

Here’s a great video from the talented people at Vox on the realities of axes in charts – something from which every strategist can take a page.

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