Quality control charts are used to monitor analytical results and check if a method is in control. They include mean (x̅) and range (R) charts to track the internal quality control sample for in run variation. Data points outside the control limits on the charts are rejected, and successive out of control results require investigation of potential biases. Trends on the charts indicate a gradual loss of reliability, while shifts represent abrupt changes, and both require troubleshooting of the analytical system. Laboratories define criteria for rejecting data based on control chart exceedances and take corrective actions such as reanalysis or material retesting when criteria are violated.

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

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

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

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)