Control Charts Flashcards
What is the purpose of control charts?
Methods for monitoring the day to day performance of an analysis or process
monitoring accuracy and precision
Control charts reveal trends in the analysis/ process, predict imminent problems & prevent invalid results from being accepted.
Control charts for variables
Used to monitor the actual measured value of some product or analytical result
Control charts for attributes
Used to monitor whether the measured value of some product or analysis lies above or below a set level.
Shewart control chart for means
A number of samples are analysed per batch of product or per day of operation of an analysis
The mean value of the samples from each batch of the product or each day of operation of the analysis is checked to see whether it lies within acceptable limits.
Shewhart control chart for standard deviations
A number of samples are analysed per batch of product of per day of operation of an analysis
The standard deviation of the samples from each batch of the product or each day of operation of the analysis is checked to see whether it lies within acceptable limits
Moving average control chart for means
A modification of the control chart for means
Highlights progressive changes or step changes more easily
Averages out natural variation before plotting values so that only significant changes are easier to see
However, the moving average delays the appearance of changes affecting the analysis
CUSUM control chart
Shows to what extent results deviate from a specified value
A number of samples are analyzed per batch of product or per day of operation of an analysis
The difference between the mean value of the samples and the specified value is determined
The difference for that day, or a batch is then added to the differences for all the previous days or batches
CUSUM charts are more sensitive to trends than Shewhart control charts.
Monitoring performance using Shewhart control charts (UWL & LWL)
The confidence limits for the UWL and LWL are typically set at the 95% confidence level.
Either the UWL or the LWL is expected to be breached by chance 1 in 20 times when the process or analysis is in control.
Monitoring performance using Shewhart control charts (UAL & LAL)
The confidence limits for the UAL and LAL are typically set at the 99.8% confidence level.
Either the UAL or the LAL is expected to be breached by chance 1 in 500 times when the process or analysis is in control.
Monitoring analysis control
If points on a Shewhart control exceed the action limits, immediate action is required - the process or analysis is out of control.
ie. instrumentation may have gone out of calibration; reagent may be out of date/ contaminated etc.
Bias in Shewhart control charts
If points are significantly more on one side of the mean than the other, there may be bias.
