Accuracy and Precision
Accuracy vs. Precision
 Accuracy: describes degree by which an experimental result
agrees with a universal standard or expected value
 Measured by error
 Precision: describes degree by which one experimental trial
agrees with other trials; also describes the number of sig figs a
measurement possesses
 Measured by deviation
Error
 Difference between an experimental result and a desired




standard
“How far you are from being perfect”
Absolute error = Experimental – Theoretical
Percent error = (Exp – Theo) / Theo x 100
Retain negative signs when calculating this
Deviation
 Difference between a trial’s result and the average of all identical
trials
 Absolute deviation = Trial value – Average
 Trial % Deviation = (Trial – Average) / Ave x 100
 Relative Deviation =
(Ave Absolute Deviation) / Ave x 100
Example
Note: All measurements in cm
Student #1
Student #2
Measurement #1
9.63
9.83
Measurement #2
10.05
9.85
Measurement #3
10.25
9.83
Measurement #4
9.52
9.83
Measurement #5
10.41
9.80
Average
9.97
9.83
•If the accepted value is 10cm, which student was more accurate?
Student #1
•Which student was more precise?
Student #2
•What was Student #1’s percent error?