1. Percent Deviation (Error)
Earth Science
Mr. Cloud

2. Percent Deviation Formula
= experimental value-accepted value x accepted value

3. Percent Deviation Example #1
A student measures the mass of an object to 127.5g and the accepted value is 125.0g
Amount of deviation = difference from accepted value
= 127.5g-125.0g
= 2.5g

4. Example #1 (continued) Percent Deviation
= exp. value-accepted value x100
accepted value
= 2.5g x 100
125.0g
= 2.0 % (always expressed as a %)

5. Example #2
A chemist measures the volume of a cube of NaCl to be 25 cm3. The accepted value is 27.5 cm3 . What is the chemist’s percent deviation?

6. Example #2
25 cm3 – 27.5 cm3 x
27.5cm3
= -9.1%
*** Percent Deviation can be negative!
*** A negative sign indicates that the experimental value is less than the accepted!