cartas de shewart iso 8258
TRANSCRIPT
Applying Trend Analysis Techniques to
Quality Control Measurements in
Radionuclide Metrology
Simon Jerome, NPL
LSUF, NPL, 24th January 2006
Event and date 2
LSUF 24th January 2006
Quality Control – ISO 17025:2005
5.9 Assuring the quality of test and calibration results
5.9.1 The laboratory shall have quality control procedures for monitoring the validity of tests and calibrations undertaken. The resulting data shall be recorded in such a way that trends are detectable and, where practicable, statistical techniques shall be applied to the reviewing of the results. This monitoring shall be planned and reviewed and may include, but not be limited to, the following:
a) regular use of certified reference materials and/or internal quality control using secondary reference materials…
NOTE The selected methods should be appropriate for the type and volume of the work undertaken.
Event and date 3
LSUF 24th January 2006
Two Techniques Commonly Used (More Are Available)
• Shewhart – more easily interpreted
• CuSum (Cumulative Summation) – more sensitive to change
Event and date 4
LSUF 24th January 2006
Use of Shewhart charts
• Common usage is to set 2σ and 3σ limits, based on 11-31 replicate results
• Action taken if 3σ limit is exceeded – expected 3 times in 1000
• Warning is registered if 2σ but not 3σ limit is exceeded – expected 5 times
in 100
• Straightforward, but no ‘warning signals’ before warning or action levels
breached
Event and date 5
LSUF 24th January 2006
• 3σ exceeded – action required
immediately, since the
procedure is out of control.
Measurement should stop
until the cause of the action
limit violation is identified
and corrected.
Shewhart Control Chart - RMG Ionisation Chambers Radium Check
Source Measurement
50.3150
50.3200
50.3250
50.3300
50.3350
50.3400
50.3450
50.3500
50.3550
50.3600
50.3650
1-Aug 3-Aug 5-Aug 7-Aug 9-Aug 11-Aug 13-Aug 15-Aug 17-Aug 19-Aug 21-Aug 23-Aug 25-Aug 27-Aug 29-Aug 31-Aug 2-Sep 4-Sep
Date
Ion
Ch
am
ber
Cu
rren
t/p
A
Event and date 6
LSUF 24th January 2006
• 2σ exceeded – situation needs
to be closely monitored
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
50 .3 15 0
50 .3 20 0
50 .3 25 0
50 .3 30 0
50 .3 35 0
50 .3 40 0
50 .3 45 0
50 .3 50 0
50 .3 55 0
50 .3 60 0
50 .3 65 0
1-Au g 3 -Au g 5 -Aug 7-Aug 9-Au g 11-Au g 13 -Au g 15 -Aug 17 -Aug 19-Au g 2 1-Au g 23 -
Au g
25 -
Au g
2 7-
Aug
2 9-
Aug
3 1-Au g 2-S e p 4-S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 7
LSUF 24th January 2006
ISO 8258:1991 – Shewhart Control Charts
• Introduces the concept of Trend Analysis
• Relies on the range observed in measurement:
rj= |x
(j+1)– x
j|
X =
R =
'
'
N
xj
j∑
'
'
)1( −
∑
N
rj
j
Event and date 8
LSUF 24th January 2006
ISO 8258:1991 – Shewhart Control Charts
• Action limit set at: X ± R×2.66
• Warning limit set at: X ± R×1.77
• ‘Information’ limit set at: X ± R×0.89
Based on a series of single observations; limits vary depending on the batch
size and subgroup
Event and date 9
LSUF 24th January 2006
• Warning Limit Exceeded
twice in under 42
measurements. Measurement
should stop until the cause of
the action limit violations are
identified and corrected. (ISO
8259:1991 suggests 2 out of 3
is an action point)
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
5 0.3150
50 .3 2 00
50 .3 2 50
50 .3 3 00
50 .3 3 50
50 .3 4 00
50 .3 4 50
50 .3 5 00
50 .3 5 50
50 .3 6 00
50 .3 6 50
1-Aug 3 -Aug 5-Aug 7-Aug 9-Au g 11-Au g 13-Au g 15-Au g 17 -Au g 19 -Au g 21-Au g 23 -
Au g
25 -
Au g
2 7 -
Au g
2 9 -
Aug
3 1-Aug 2 -S e p 4 -S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 10
LSUF 24th January 2006
• Nine points in a row on one
side of the mean. This may
indicate that there has been a
shift in the mean; work can
continue, however the cause
of the shift needs to be
identified and the limits re-set
if justified
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
5 0 .3 15 0
5 0 .3 2 0 0
5 0 .3 2 5 0
5 0 .3 3 0 0
5 0 .3 3 5 0
5 0 .3 4 0 0
5 0 .3 4 5 0
5 0 .3 5 0 0
5 0 .3 5 5 0
5 0 .3 6 0 0
5 0 .3 6 5 0
1-Au g 3 -Au g 5 -Au g 7 -Au g 9 -Au g 11-Au g 13 -Au g 15 -Au g 17 -Au g 19 -Au g 2 1-Au g 2 3 -
Au g
2 5 -
Au g
2 7 -
Au g
2 9 -
Au g
3 1-Au g 2 -S e p 4 -S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 11
LSUF 24th January 2006
• Six points in a row steadily
increasing or decreasing.
Suggests that the
measurements are shifting
with time; needs investigation,
since the action and warning
limits will eventually be
exceeded
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
50 .3150
50 .3200
50 .3250
50 .3300
50 .3350
50 .3400
50 .3450
50 .3500
50 .3550
50 .3600
50 .3650
1-Aug 3-Aug 5-Aug 7-Aug 9-Aug 11-Aug 13-Aug 15-Aug 17-Aug 19-Aug 21-Aug 23-
Aug
25-
Aug
27-
Aug
29-
Aug
31-Aug 2-S e p 4 -S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 12
LSUF 24th January 2006
• Fourteen points in a row
alternating up and down.
Suggests a cyclical change in
conditions; may be due to two
different operators
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
50 .3150
50 .3200
50 .3250
50 .3300
50 .3350
50 .3400
50 .3450
50 .3500
50 .3550
50 .3600
50 .3650
1-Aug 3-Aug 5-Aug 7-Aug 9-Aug 11-Aug 13-Aug 15-Aug 17-Aug 19 -Aug 21-Aug 23-
Aug
25-
Aug
27-
Aug
29-
Aug
31-Aug 2-S e p 4 -S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 13
LSUF 24th January 2006
• Four out of five points in a
row exceeding the information
level. This may indicate a
mean shift
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
5 0 .3 15 0
5 0 .3 2 0 0
5 0 .3 2 5 0
5 0 .3 3 0 0
5 0 .3 3 5 0
5 0 .3 4 0 0
5 0 .3 4 5 0
5 0 .3 5 0 0
5 0 .3 5 5 0
5 0 .3 6 0 0
5 0 .3 6 5 0
1-Au g 3 -Au g 5 -Au g 7 -Au g 9 -Au g 11-Au g 13 -Au g 15 -Au g 17 -Au g 19 -Au g 2 1-Au g 2 3 -
Au g
2 5 -
Au g
2 7 -
Au g
2 9 -
Au g
3 1-Au g 2 -S e p 4 -S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 14
LSUF 24th January 2006
• Fifteen points in a row
between the lower and upper
information levels. This may
arise after the procedure has
been operated for some time
and may require the limits to
be re-set
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
50 .3150
50 .3200
50 .3250
50 .3300
50 .3350
50 .3400
50 .3450
50 .3500
50 .3550
50 .3600
50 .3650
1-Aug 3-Aug 5-Aug 7-Aug 9-Aug 11-Aug 13-Aug 15-Aug 17-Aug 19-Aug 21-Aug 23-
Aug
25-
Aug
27-
Aug
29-
Aug
31-Aug 2-S e p 4 -S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 15
LSUF 24th January 2006
• Eight points in a row either
side of the mean with none
between the upper and lower
information levels. This may
indicate a mean shift.
Shewhart Control Chart - RMG Ionisation Chambers Radium Check Source
Measurement
5 0 .3 15 0
5 0 .3 2 0 0
5 0 .3 2 5 0
5 0 .3 3 0 0
5 0 .3 3 5 0
5 0 .3 4 0 0
5 0 .3 4 5 0
5 0 .3 5 0 0
5 0 .3 5 5 0
5 0 .3 6 0 0
5 0 .3 6 5 0
1-Au g 3 -Au g 5 -Au g 7 -Au g 9 -Au g 11-Au g 13 -Au g 15 -Au g 17 -Au g 19 -Au g 2 1-Au g 2 3 -
Au g
2 5 -
Au g
2 7 -
Au g
2 9 -
Au g
3 1-Au g 2 -S e p 4 -S e p
Date
Ion
Ch
am
ber C
urren
t/p
A
Event and date 16
LSUF 24th January 2006
In practice.
• Use software to record and present the data
• Require operators to manually carry out a check (use a paper-based check
list) for the above conditions
• Carry out investigations as necessary and institute improvement actions
However, it is incorrect simply to:
• Repeat a measurement and find the chart to be ‘in control’
• Reset the chart parameters without at least seeking a reason
• Rely on the software to do it for you
• Rely on manufacturer’s software
Event and date 17
LSUF 24th January 2006
In addition:
• Use a CuSum Chart:
c1 = x1 – X
c2 = c1 + x2 – X
c3 = c2 + x3 – X
.
.
.
ci = ci-1 + xi – X
• Deviation from control become clear very rapidly
Event and date 18
LSUF 24th January 2006
Shewhart Value Control Chart for TriCarb
64.3
64.5
64.7
64.9
65.1
65.3
65.5
65.7
65.9
66.1
30-Ju
n-03
19-A
ug-0
3
08-O
ct-0
3
27-N
ov-0
3
16-Ja
n-04
06-M
ar-0
4
25-A
pr-0
4
14-Ju
n-04
03-A
ug-0
4
22-Sep
-04
11-N
ov-0
4
31-D
ec-0
4
19-Feb
-05
10-A
pr-0
5
30-M
ay-0
5
19-Ju
l-05
Date
Tri
tiu
m c
ou
nti
ng
eff
icie
ncy
(%
)
Ev
ent an
d d
ate1
9
LS
UF
24
thJ
an
ua
ry 2
006
Cu
Su
m fo
r Tritiu
m e
fficie
ncy
-25.0
000
-20.0
000
-15.0
000
-10.0
000
-5.0
000
0.0
000
5.0
000
30-Jun-03
19-Aug-03
08-Oct-03
27-Nov-03
16-Jan-04
06-Mar-04
25-Apr-04
14-Jun-04
03-Aug-04
22-Sep-04
11-Nov-04
31-Dec-04
19-Feb-05
10-Apr-05
30-May-05
19-Jul-05
Date
CuSum Tritium efficiency
Event and date 20
LSUF 24th January 2006
Tritium Check Source for LSC
• Shewhart Chart gives first warning on 6th November – 9 points in a row,
one side of the mean – (and many thereafter)
• CuSum Chart gives first warning around 1st November – problem is clear
from this point on
• On-board Q/C software gave first fault message on 30th January – ‘3σ’
limit violation
Event and date 21
LSUF 24th January 2006
Shewhart Value Control Chart for TriCarb
8.600
9.100
9.600
10.100
10.600
11.100
11.600
12.100
12.600
13.100
13.600
30-Ju
n-03
19-A
ug-0
3
08-O
ct-0
3
27-N
ov-0
3
16-Ja
n-04
06-M
ar-0
4
25-A
pr-0
4
14-Ju
n-04
03-A
ug-0
4
22-Sep
-04
11-N
ov-0
4
31-D
ec-0
4
19-Feb
-05
10-A
pr-0
5
30-M
ay-0
5
19-Ju
l-05
Date
Tri
tiu
m b
ack
gro
un
d
(co
un
ts/s
eco
nd
)
Ev
ent an
d d
ate2
2
LS
UF
24
thJ
an
ua
ry 2
006
Cu
Su
m fo
r T
ritium
Ba
ck
gro
un
d
-10.0
000
0.0
000
10.0
000
20.0
000
30.0
000
40.0
000
50.0
000
60.0
000
30-Jun-03
19-Aug-03
08-Oct-03
27-Nov-03
16-Jan-04
06-Mar-04
25-Apr-04
14-Jun-04
03-Aug-04
22-Sep-04
11-Nov-04
31-Dec-04
19-Feb-05
10-Apr-05
30-May-05
19-Jul-05
Date
CuSum Tritium Background
Event and date 23
LSUF 24th January 2006
Tritium Background for LSC
• Shewhart Chart gives first warning on 15th December – action limit
violation – (event on 8th October assigned to 99Mo/99mTc source in
vicinity)
• CuSum Chart gives first warning around 18th December – problem is clear
from this point on (event on 8th October does not affect CuSum)
• On-board Q/C software gave first fault message on 15th February – ‘3σ’
limit violation
Event and date 24
LSUF 24th January 2006
Analysis of data
• Engineer called out in February
• Deteriorating Photo-Multiplier tube identified
• 14C check source did not return comparable data, apart from on action limit
violation that did not have an assignable cause
• Subsequent measurements with other systems indicate general instability
of 3H measurements – observed at other establishments
• Further investigations reveal ‘hidden’ parameters that low energy β-
emitters are very sensitive to (such as DBB)
• Essential to know the system well before use
Event and date 25
LSUF 24th January 2006
Conclusions
• Full implementation of ISO 8258:1991 allows operators to far better
understand measurement systems
• Potential faults can be trapped by preventative actions rather than
corrective actions
• Simple to use and interpret
• Applicable to similar systems – α-spectrometers, Germanium γ-
spectrometers, NaI(Tl) γ-spectrometers, ionisation chambers &c
• Customer calibrations and source generation can be carried out with
greater confidence
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