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Control Charts A Benchmark of Quality
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What is a Control Chart?
A statistical tool used in quality control to:
o Analyze and understand variability for a particular method
o Determine a method’s consistency and capability
o Monitor effects of the variability for a method on the difference between
target and actual performance.
o Limits may be adjusted to allow for more or less variability.
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Anatomy of a Control Chart:o Y-Axis: The unit of measurement for the process (i.e. %, ppm, etc.)
o X-Axis: The date; usually spanning days, months, or years
o Black center line: The average of the process’ compiled data
o Thin black line: ± 1 standard deviations from the average
o Blue dotted line: ± 2 standard deviations from the average
o Red line: ± 3 standard deviations from the average
o The red line represents the upper and lower control limits (UCL and LCL)
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13.00Average:
Variable2Variable3Variable4Variable5Variable6Variable7Variable8+1Stdev:
-1StDev:
+2StDev:-2StDev:
+3StDev:
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Assessing the Data
o Data will naturally have variation based on many variables (i.e. human error,
control degradation, accuracy of method)
o Want data to stay within ± 2 standard deviations of average
o Do not want data to go outside of ± 3 standard deviations or it will be
outside the upper or lower control limits (Red Line)
o After 20 tests have been completed, the limits may be set for a chart by
Quality or relevant personnel
o Control charts should be reviewed by analysts daily upon entering data and
by Technical Managers and QUA at minimum every two months
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Assessing the Data (Part 2)o Certain patterns of data require special attention if found:
o Trends: 6 or more consecutive points increasing or decreasing
o Biases: 8 or more consecutive points on the same side of the center line
o Oscillatory Trend: 14 or more points oscillating back and forth
o These situations do not pose an immediate problem, but should be
monitored. If the process does not correct itself:
o Notify supervisor
o Initiate OOS investigation to determine the cause
o Hold or recall data depending on findings of OOS investigation
o Document OOS investigation and any preventive action
o Requires the interaction of the Technical Manager, QUA, laboratory
management and other relevant personnel
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Duplicate Control Charts
o Two preparations of one sample using the same method with identical
conditions by the same analyst using the same equipment within a short
time interval (same day)
o Provides a measure of method precision/repeatability
o Monitor the difference in the analyzed value of the duplicate by plotting
the results.
o Ensure analysis meets method requirements.
o Duplicates serve as a way for a lab to demonstrate reproducibility by our
analysts for our methods.
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Relative Standard Deviation
o An RSD (%) shows the analytical variance of a particular test method,
regardless of the sample
o The RSD is already calculated in the control charts and changes as more
data is entered automatically
o Depending on the method, higher or lower RSD’s are accepted for control
charts. (i.e. Enzymes are allowed more variability because enzymatic
activity is not always consistent)
o For most methods, an RSD of >10% is a red flag. It shows too much
variability for that method
o To calculate an RSD:
o RSD% = (Standard Deviation/Average)*100
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Measurement of Uncertainty
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