QC charts - how to interpret the results of internal QC: QC charts and development of estimates of Measurement of Uncertainty using a top down approach - Rob De Hayr, GLOSOLAN Vice-Chair 4th Asian Soil Laboratory Network (SEALNET) meeting (online), 30 June - 2 July 2020

1. Quality Control Charts
Rob De Hayr

2. Section 5.9.2
quality control data analysed and when found to
be outside of predefined criteria, planned action
shall be taken to correct the problem and prevent
incorrect results from being reported
ISO/IEC 17025 Requirements
ISO/IEC 17025:2005
http://santia.co.uk/wp-
content/uploads/2017/10/P405-ISO-IEC-17025-
2005-Standard.pdf

3. Quality Control Charts
Mean and Range (x̅ and R chart)
• Monitor results for Internal Quality Control Sample
• Criteria used to check that a method is “in control”
• Target value (mean), upper and lower limits
• In run variation with limits
• The control chart can also show trends in results
caused by biases not large enough to put the result
out of range.

4. Quality Control Charts
Mean Chart
Date/Batch

5. Date/Batch
RangeofDuplicates
Quality Control Charts
Range Chart

6. Quality Control Charts
Mean and Range (x̅ and R chart)
• Initial limits are calculated using a minimum of 10 points.
• Recalculated using 20 points
• Final calculation using over ≥ 40 data points.
Systemic statistical control is required for reliable estimates
of chart limits
• indicated by the scatter of points on the control chart,
• analysis of secondary and/or certified reference
materials to confirm accuracy

7. Criteria for rejecting data
• Outside upper or lower warning limit: Data require
careful inspection but may be acceptable – see lab
manager
• Outside upper or lower control limit: Data are rejected
and run repeated
• Two successive results beyond the same warning limit:
Data are rejected and reason for bias is investigated
• 6 -10 successive results on the same side of the mean:
Data are rejected and reason for bias is investigated.
• Five or more points moving in the same direction: Trend
needs to be investigated.

8. Criteria for rejecting data
If any of the rejection rules are violated the following
actions should be taken:
• Repeat the analysis, if the next point is satisfactory, continue the
analysis. If not, then
• Investigate the cause of the exceeding.
• Do not use the results of the batch, run, day or period
concerned until the cause is trailed. Only use the results if
rectification is justified (e.g. when a calculation error was made).
• If no rectification is possible, after elimination of the source of
the error, repeat the analysis of the batch(es) concerned. If this
next point is satisfactory, the analysis can be continued.

9. Examples of charts

10. Criteria for rejecting data
Trend
A trend indicates a gradual loss of reliability in the test system.
Trends are usually subtle.
Causes of trending may include:
• Gradual deterioration of calibration standards or reagents
• Deterioration of the instrument light source or light filter
• Gradual accumulation of debris in sample/reagent tubing or
electrodes
• Gradual deterioration of control materials

11. Criteria for rejecting data
Shift
Abrupt changes in the control mean are defined as shifts.
Shifts may be caused by:
• Inaccurate calibration/recalibration
• Sudden failure or change in the light source
• Error in standard preparation
• Change in reagent formulation or reagent lot (quality)
• Major instrument maintenance
• Change in room temperature or humidity
• Failure in the sampling system
• Failure in reagent dispense system
• Change of control materials

12. Outside upper or
lower warning limit
Outside upper or
lower control limit
6 -10 successive results
on the same side of the
mean
Trend Shift

13. QC Scoring
at Chemistry Centre Laboratories, Qld, Australia
• If QC score for batch (run) is 3 or more, then
analyst/instrument operator consults with
authorised signatory re:
• Authorise batch ?
• Take remedial action ?

14. QC Failure Demerit Points Notes if total score exceeds 2
Single Blank > 2 X PQL 1 -
Two Blanks > 2 X PQL 3 Results may be used if blank
subtraction is a possibility.
Senior Officer to advise. QC
investigation is mandatory.
Duplicate > 1.0 X Uncertainty 1 May be accepted if only one
out of multiple duplicates fail
within a batch
Matrix Spike Recovery > ±
20% for ultra trace analytes
1 May be accepted if only one
out of multiple duplicates fail
within a batch
Matrix Spike Recovery > ±
10% for non ultra trace
analytes
1 May be accepted if only one
out of multiple duplicates fail
within a batch
Any individual QC sample >
action limit
2 Senior officer to advise
Two QC samples within the
same batch > Action limit
4 Senior officer to advise
3 consecutive QC samples
outside warning limits
3 Senior officer to advise

15. Thanks for your attention
https://asq.org/quality-resources/control-chart
Further Reading and resources
http://www.fao.org/3/w7295e/w7295e0a.htm#8.3.2%20co
ntrol%20chart%20of%20the%20mean%20(mean%20chart)