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Control Charts
Confidence Interval
Student’s “t” test
How do we verify accuracy?
Quality Control Charts
μ
is known or
assumed
!
is known
–
3
!
action line:
1 in 500
–
2.5 s action
1 in 100 (99%)
–
2
!
warning:
1 in 20 (95%)
Confidence Interval
•
The standard deviation indicates
precision
–
varies with N
•
How close is experimental mean to
μ?
–
Theory allows an estimate of the range
within which the true value might fall with a
given probability
–
The range is called “confidence interval”
–
limits of the range are the “confidence
limits”
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t  statistical factor, depends on N and the
confidence level desired
•
confidence limit is product of t and the
standard deviation of the mean
•
Values of t given in tables for different
confidence levels and degrees of freedom
Student’s “t” Table
range increases with required confidence level
range
decreases
"
(N)
Example for Confidence limits
•
A soda ash sample is analyzed in the lab.
by titation with standard HCl
•
Triplicate determination of Na
2
CO
3
•
93.50, 93.58, 93.43%
•
Within what range of confidence are you
95% sure that the true value lies?
Solution for Confidence Limits
•
mean = 93.5%, s = 0.075% Na
2
CO
3
•
At 95% confidence and 2 degrees of
freedom, t = 4.303
– Confidence limit
Notes on Confidence Limits
•
true value falls with range 93.31 and
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This note was uploaded on 02/25/2012 for the course CHEM 232 taught by Professor Michel, r during the Fall '08 term at UConn.
 Fall '08
 Michel, R

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