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?