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Control Charts A Benchmark of Quality

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.

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:

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

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

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.

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

Measurement of Uncertainty

Thank You!