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10 April 2013

The Difference between Accuracy and Precision

In technology and science, accuracy and precision are different, although they go together.

For the measured characteristic
  • Precision is described by the range of values.
  • Accuracy is described by the difference between the average value and the specified value.
Lets have a goal of making two batches of cookies and specify that they measure 5" across. Afterwards, we measure the cookies.

Chocolate cookie widths
  • Average = 5.10 "
  • Range = 0.25"
Cinnamon cookie widths
  • Average = 5.01"
  • Range = 0.50"
The chocolate cookies have a smaller range of sizes, so they are more precise. The average size of the Cinnamon cookies comes closer to the specified value, though, so they are more accurate.
Does that seem counterintuitive?
For a measurement or a measuring tool
  • Precision is described by how many significant figures the tool gives you.
  • Accuracy is described by how closely the measurement matches the value of a standard device and also by how accurate the standard is.
As another example, I have two 18" rulers.
  • One came from a store that sells drafting supplies. It is graduated in 16ths of an inch.
  • The other, I made for myself after I loaned the store-bought ruler to a neighbor kid. I based it on one cubit (18"), the length from my elbow to my finger tips. When I graduated the cubit ruler in 50ths of an inch, I eyeballed the measurements and marked it by hand.
 The cubit ruler is more precise, but the store-bought ruler is more accurate.

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