Accuracy and Precision

They mean slightly different things!

Accuracy

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

Precision

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

 

Examples of Precision and Accuracy:

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!

Bias (don't let precision fool you!)

If you 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 will be wrong by the same amount. That is bias.

Degree of Accuracy

Accuracy depends on the instrument you are measuring with. But as a general rule:

The degree of accuracy is half a unit each side of the unit of measure

Examples:

If your instrument measures in "1"s
then any value between and is measured as "7"
If your instrument measures in "2"s
then any value between 7 and 9 is measured as "8"

(Notice that the arrow points to the same spot, but the measured values are different!
Read more at Errors in Measurement. )