Quality Assurance and Control Charts
Practise Quality Assurance and Control Charts for GCSE Statistics with this free worksheet and full mark scheme — Higher tier exam-style questions with worked answers for AQA and Edexcel. Control charts monitor a process over time using warning and action limits to check for consistency.
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These worksheets and mark schemes are original, written for Virtus Academy and checked against the current AQA and Edexcel specifications. Every worksheet comes with a full mark scheme.
This is a Higher tier only topic, so there's no Foundation paper — only the Higher worksheet and mark scheme below.
Topic overview
Quality assurance uses control charts to monitor a production process and detect when it is going wrong. This is a Higher-only topic.
A control chart plots a sample statistic, usually the mean or the range, against time. It has a target value line and two pairs of limits: warning limits at two standard deviations from the target, and action limits at three.
A point between the warning and action limits is a signal to monitor the process more closely. A point beyond an action limit means the process should be stopped and investigated. A run of points on the same side of the target, even within the limits, also suggests the process has drifted and needs attention.
Revision notes
The chart
A sample statistic — usually the mean or the range — is plotted against time.
The chart shows a target value line, warning limits at two standard deviations from the target, and action limits at three standard deviations.
Interpreting points
Within the warning limits: the process is operating acceptably.
Between warning and action limits: monitor more closely and take another sample. Beyond an action limit: stop the process and investigate, as it is no longer operating correctly.
Runs and trends
A run of consecutive points on the same side of the target suggests the process has drifted.
This is a signal even when every point lies within the limits, because random variation would produce points on both sides. A steady trend towards a limit is the same warning.
Key points
- Control charts monitor a production process.
- A sample statistic is plotted against time.
- Warning limits are at two standard deviations.
- Action limits are at three standard deviations.
- Beyond an action limit the process is stopped.
- A run on one side suggests drift.
Worked examples
Example 1
A point falls beyond the upper action limit. State what should happen. [1 mark]
Working
Example 2
A point falls between the warning and action limits. State what should happen. [2 marks]
Working
Example 3
Eight consecutive points lie above the target value but within the warning limits. Explain why this is a concern. [2 marks]
Working
Common mistakes
Confusing warning and action limits.
Warning is at two standard deviations, action at three.
Ignoring runs within the limits.
A run on one side signals drift even when no limit is crossed.
Stopping the process at a warning limit.
Monitor more closely; stop only beyond an action limit.
Plotting individual values rather than a sample statistic.
Control charts plot the sample mean or range.
Exam tips
- Learn both limit positions precisely.
- Match the action to which region the point falls in.
- Watch for runs as well as individual points.
- Remember this is a Higher-only topic.
Key terms
- Control chart
- A chart monitoring a process over time.
- Warning limit
- A limit at two standard deviations from the target.
- Action limit
- A limit at three standard deviations from the target.
- Run
- Consecutive points on the same side of the target.
Related topics
Written and reviewed against the current AQA and Edexcel specifications. Spotted an error? Let us know.