Accuracy and precision are the two measures of the reliability of an analytical result. People often interchange these terms thinking that they are the same.

Accuracy is the degree with which the obtained result agrees with the actual result and precision is the ability to generate the same result in repeated tests of the same sample.

The simplest method to understand the concept of accuracy and precision is to think of a target shooter trying to hit a bulls eye.

• A highly accurate and precise shooter sees all his shots grouped in the bulls eye of his target.
• The shooter that is not very accurate but his shooting is highly precise sees all his shots grouped together but well away from the bulls eye. This shooter may have “pulled” the gun in the same direction each time he shot creating a systematic error or bias.
• The shooter that has high accuracy but low precision sees his shots evenly distributed within the target rings
• The shooter with low accuracy and low precision sees his shots scattered over the target with many shots completely missing the target.

A laboratory should be aiming for both highly accurate and highly precise results.