How Many Decimal Places Are Helpful?

Sep 22nd, 2015 / Data Visualization / , ,

How many decimal places does your graph actually need?
2, 1 or 0 decimal places?
Most of my projects are in the social sciences. Social science measurement is not exact. We’re often creating instruments and tools from scratch to gather information. There are no rulers, tape measures, thermometers, or fancy gadgets guaranteed to give us exact numbers with exact certainty.
Furthermore, I’m often involved in the data collection process. We’re always going to be missing a few surveys. We’re always going to need to estimate a few numbers. That’s how data collection works in the real world.
I would never recommend that someone make a decision based on two decimal places.
Should Country A be awarded more funding because they reached 89.34% versus 89%?
Should Country B’s intervention program get revised because they reached 49.71% versus 50%?
Does it really matter that Country C reached 25.07%? Can’t we simply round down to 25%?
The last thing I want to do is give decisionmakers a false sense of precision.
 
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In real world datasets, I rarely show any decimal places.
This revised graph would score well on the graph has appropriate level of precision section of the Data Visualization Checklist.
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Ready to revise your own graphs?
Do not manually round your numbers up or down. You’re bound to make a rounding error or typo. Or worse, you’ll probably want to bang your head against the wall! Rounding by hand is tedious and completely unnecessary.
Instead, use the Add Decimal and Decrease Decimal buttons on your Home tab to automatically adjust the number of decimal places that are displayed. Since your data table is linked to your graph, the graph will get instantly adjusted.
The raw numbers – like that 9.84% – are still there. They’re not lost forever. They’re simply hidden from view.
 
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Are there instances in which you’d intentionally keep 1 or 2 decimal places in your graphs? Share your opinion below.
 
 

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