Measurement ReliabilityPrecision & Accuracy
Precision
The degree to which several measurements provide answers very close to each other.
Indicator of the scatter in the data.› The less the scatter, The higher the
precision.
Accuracy The nearness of a measurement
to the standard or actual value.
Precision & Accuracy
Precision Assesses data with many
measurements and no accepted (target) value
Quantified using uncertainty› Largest deviation from mean› Smaller deviations, better
precision
Accuracy Assess one measurement or many
measurements
Quantified with % error› (experimental) – (accepted) X 100
(accepted)
Accurate or Precise? Baseball player hits the ball directly
to the shortstop every time at bat.
Darts player hits the bulls eye with every shot.
Precise
Precise & Accurate
Accurate or Precise? Soccer player hits the right goal post
with every shot on goal.
Basketball player sometimes makes a basket, sometimes bounces off the rim, sometimes hits the back board, and sometimes shoots straight over the back board.
Precise
Not Precise Or Accurate
Errors: Random Errors Unnoticed variations in technique, tiny
changes in the experimental environment, etc.
Affect precision.
Truly random effects average out over a large number of trials.
Errors: Systematic Errors Mistake which does not change during
the experiment.
Affect accuracy, not precision.
Measurement: Ruler
Precision & Accuracy
Uncertainty Calculations Calculate mean Calculate difference between each measurement
and the mean (the deviation) Use the largest of these values as the uncertainty The smaller the range,
the more precise.
trial 1 trial 2 trial 3 trial 4 trial 5 mean
data 0.748 0.764 0.763 0.741 0.749 0.753
(data - mean) -0.005 0.011 0.01 -0.012 -0.004 +/- 0.012
Uncertainty Calculations Calculate mean Calculate difference
between each measurement and the mean (the deviation)
Use the largest of these values as the uncertainty
Value in range of 0.741 to 0.765
The smaller the range, the more precise.
data (data - mean)
0.748 -0.005
0.764 0.011
0.763 0.01
0.741 -0.012
0.749 -0.004
0.753 +/- 0.012