Accuracy and Precision "You don’t want one without the other!"

Accuracy and Precision "You don’t want one without the other!"

If you put the search terms “Accuracy Definition” into Google you will be presented with the following result:
“the quality or state of being correct or precise”

That is true, but when looking at it from a Metrologist’s standpoint the second definition is more correct:
“The degree to which the result of a measurement, calculation, or specification conforms to the correct value or a standard.”
So what is wrong with the first statement?  It is the use of the word precise.  I will briefly go over the differences between Accuracy and Precision (aka Repeatability).

People assume ACCURACY is the most important characteristic of measuring equipment and I am not disagreeing with them.  However, an accurate gage is not necessarily the same thing as a repeatable or precise gage.

Take a look at the image below that has been used in a similar fashion in many textbooks.  It uses a target with bullet holes in it to represent some type of gage taking measurements on a test piece.  You could also use a shuffleboard, dart board or similar to show the comparison.

The Red Grouping shows a pattern closer to the center of the target or true value of a test piece, hence a more “accurate” result.

The Blue Grouping shows a less accurate result because it is farther away from the center of the target (or desired measurement) but the pattern is much tighter.  This represents a more PRECISE or REPEATABLE gage. 

From a Metrologist’s standpoint if the equipment that produced the Blue Grouping can be calibrated/adjusted to correct accuracy problem i.e. datuming a height gage on a surface plate, cleaning the jaws of a caliper, remastering a bore gage in a setting ring or adjusting the sleeve of a micrometer with a mic wrench, it is simply the better gage.

Please let us know if you have any questions or comments.  MEASURE SMARTER!