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