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We often display percentages which sum is expected to equal 100%. For example, in a pie chart. These percentages are sometimes rounded, either for clarity (25% instead of 25.434%) or because we have to (67% instead of 66.66666...%).

However, this can lead to inconsistencies. For example, take a 1/3-1/3-1/3 situation. We have three percentages, all equal to 33.333333...%. These percentages have to be rounded, whatever it is 33.33% or simply 33%. But 33% + 33% + 33% is equal to 99%, not the expected 100%.

I see two solutions:

  • Just use 33%. It is up to the user to realize that the mistake is due to rounding issues.
  • Adjust one of the percentage: 33%, 33% and 34%. The sum is equal to 100%, which is satisfying. However, the percentages are not identical, which is somehow misleading.

What are the best practices?

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  • 33/33/34 wrongly gives 34 the priority.
    – bloodyKnuckles
    Feb 3 at 9:37
  • Rounding to integers is part of the problem. Can you round to a decimal place or two?
    – bloodyKnuckles
    Feb 3 at 9:41
  • 33/33/34 is clearly wrong. However it might not happen that often in real life examples. Let's say we figures in the hundreds, a strict 1/3 - 1/3 - 1/3 is unlikely. So 33/33/34 might appear as a rare, non-harmful bug. I don't pretend this is the right solution (I would not have posted this question in the first place), I just highlight it might be.
    – philippe_b
    Feb 4 at 8:56
  • @bloodyKnuckles Using a few decimals just moves the issue, and make it less relevant. 33.3% / 33.3% / 33.4% is still wrong, but less. It will probably be easier for the user to ignore the problem.
    – philippe_b
    Feb 4 at 9:17
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    I think giving even one decimal is enough for people to figure it out / feel resolved about it not adding up. As soon as they know they're dealing with some sort of precision it's a different ballgame. 33 + 33 + 33 feels very different from 33.3 + 33.3 + 33.3.
    – Luke Sawczak
    Feb 5 at 20:24

1 Answer 1

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When a decimal value is encountered, add a plus sign instead of inconsistent round, or including the decimal value:

"pie" chart

  • Pumpkin: 33%+
  • Apple: 33%+
  • Lemon Meringue: 33%+
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  • 1
    Or alternatively: ~33%
    – Tim
    Feb 7 at 17:26
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    Some, maybe obvious, distinction between + and ~, 33+ means 'more than 33', and ~33 means 'around 33', or 'approximately 33', not as precise. ~ is similar to +/-.
    – bloodyKnuckles
    Feb 8 at 16:27

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