The Difference Between Precision and Accuracy

Never trust on it!Much of the work that I do has all sorts of approximations and guesses.  Most of it is done on a computer and one really nice thing about a computer is that it will generate a great degree of precision.  Want to know how many Watts that new device is going to use before it’s ever been made? The computer will tell you 12.483028605 Watts.  Wow, that’s really precise — but there is no way the estimation can be that accurate.

The problem is that we have a hard time distinguishing between the two.  The value 12.483028605 Watts sound like we really know a lot more than we do.  Compare that to saying about 12 Watts and now we are closer to the truth.  Excel spreadsheets encourage this delusion.  Create some guesses about your future revenue for the company, build a formula and tell it to apply the formula for 5 years.  Viola! In 5 years, we’ll be making $312,382.12.  Super precise — not very accurate.

It’s important whenever we see values explained this way that we separate reality from the illusion.  Just because we see lots of digits after the decimal point does not mean we know to that level of accuracy. We should pause on many values that we come across and ask whether it’s really possible to know the value with that degree of certainty.  Also, when we communicate with others, we can help by only presenting the precision of the value to correlate with its proper accuracy (12 watts, about $300k or so).

I’m 98.28301756% sure that doing this will improve all of our lives.

[Picture courtesy:  Pulpolux !!!]






One response to “The Difference Between Precision and Accuracy”

  1. Bill Gascoyne Avatar
    Bill Gascoyne

    I have a favorite cartoon that shows the difference well. It features a disappointed Robin Hood-ish character gazing at a target with a tight cluster of arrows in the far top left of the target.

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