Titra lab, Standard deviation calculation – Hach-Lange TITRALAB 840_845 Reference Manual User Manual
Page 208
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Titra
Lab
®
TitraLab 840 and 845 Reference Manual
Page 208
3. Standard deviation calculation
For every result obtained on several aliquots, a mean value is calculated with which a standard
deviation is associated.
Definitions:
R
i
= Result of test i
Mean result:
Standard deviation on the mean value
σ
µ
:
- Number of tests N = 2 to 5 : estimation of the Q variance for a small number of tests.
with R
max
= maximum value of the R
i
and R
min
= minimum value of the R
i
:
- Number of tests N > 5
Result is expressed with the mean value R and the standard deviation
σ
µ
on the mean value:
Result = R ±
σ
µ
N
Q
2
1.128
3
1.693
4
2.059
5
2.326
¦
=
n
i
n
R
SR
1
¦
=
n
i
n
R
SR
1
2
2
N
SR
R
n
=
N
Q
R
R
2
min
max
2
¸¸¹
·
¨¨©
§
−
=
µ
σ
(
)
N
N
N
SR
SR
n
n
µ
×
−
−
=
1
2
2
2
σ
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