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In most cases a measurand, now called Y, is not measured directly, but is
determined from
N
other input quantities
X
1
,
X
2
,
….
,
X
N
, through a functional relationship
f
:
Combined uncertainty
The functional relationship
f
is also referred to as the
model equation for the measurand.
1 2
N
Y f ( X , X ,.....X )
=
The set of input quantities
X
1
,
X
2
, ...,
X
N
may be differentiated into two
following categories:
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