Accuracy and Precision

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Accuracy and Precision

They mean slightly different things!


Accuracy is how close a measured value is to the actual (true) value.


Precision is how close the measured values are to each other.

Examples of Precision and Accuracy:

low accuracy hi precisionhi accuracy low precisionhi accuracy hi precision
Low Accuracy
High Precision
High Accuracy
Low Precision
High Accuracy
High Precision
So, if you are playing soccer and you always hit the left goal post instead of scoring, then you are not accurate, but you are precise!

How to Remember?

  • aCcurate is Correct (a bullseye).
  • pRecise is Repeating (hitting the same spot, but maybe not the correct spot)

Bias (don't let precision fool you!)

When we measure something several times and all values are close, they may all be wrong if there is a "Bias"
Bias is a systematic (built-in) error which makes all measurements wrong by a certain amount.

Examples of Bias

  • The scales read "1 kg" when there is nothing on them
  • You always measure your height wearing shoes with thick soles.
  • A stopwatch that takes half a second to stop when clicked
In each case all measurements are wrong by the same amount. That is bias.

Degree of Accuracy

Accuracy depends on the instrument we are measuring with. But as a general rule:
The degree of accuracy is half a unit each side of the unit of measure


When an instrument measures in "1"s
any value between  and  is measured as "7"
7 plus minus 0.5
When an instrument measures in "2"s
any value between 7 and 9 is measured as "8"
8 plus minus 1
(Notice that the arrow points to the same spot, but the measured values are different!)

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