Fluke Digital Multimeter - Page 53

©Fluke Calibration 2011 Basics Of Measurement Uncertainty for DMM Calibration 53
c
m
ku
U

Calculating the expanded
uncertainty
Level of
Confidence
(percent)
Coverage
factor
k
68.27% 1
90% 1.645
95% 1.960
95.45% 2.0
99% 2.576
99.73% 3
k
is the coverage factor
• How confident should you be with your measurement results?
(68%, 95%, 99%....)
• 95% confidence is commonly accepted as appropriate.
• U
m
expresses the uncertainty, expanded from a single standard
deviation of 68%, to uncertainty value with a higher confidence.
• For a large population with a normal distribution, 95% coverage
is calculated by k with a value of 1.96
(or sometimes 2 for convenience – giving 95.45%)