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Interpreting Results and Evaluating FindingsControl ChartsStatistical Process ControlQuality ControlGCSE
AQA GCSE · Question 12.1 · Interpreting Results and Evaluating Findings
In a factory, three different machines, A, B and C, put crisps into bags. Filled bags are labelled as having a mass of 25g. Every two days, Luka, the quality control manager, obtains a sample of filled bags from each machine and calculates the sample mean. The control chart for 10 consecutive large samples for Machine A is shown below. What should Luka do, if anything, with Machine A, based on this chart?
In a factory, three different machines, A, B and C, put crisps into bags. Filled bags are labelled as having a mass of 25g. Every two days, Luka, the quality control manager, obtains a sample of filled bags from each machine and calculates the sample mean. The control chart for 10 consecutive large samples for Machine A is shown below. What should Luka do, if anything, with Machine A, based on this chart?
How to approach this question
1. **Understand Control Chart Rules**: A process is generally considered "in control" if:
* All points are within the action limits.
* There are no suspicious patterns (e.g., two consecutive points between warning and action limits, a long run of points on one side of the target, a clear trend towards a limit).
2. **Examine the Chart for Machine A**:
* Are any points outside the action lines? No.
* Are any points outside the warning lines? No.
* Are there any clear trends or patterns? No, the points are randomly scattered around the target mass.
3. **Formulate a Conclusion**: Since no rules have been violated, the process is in control. Therefore, no action is required. Luka should continue to monitor the machine.
Full Answer
Luka should do nothing, as the process is in control. All points are within the action limits and there are no suspicious patterns.
A control chart is used to monitor a process over time. Action is typically required if a data point falls outside the action limits, or if there is a non-random pattern of points (e.g., two consecutive points falling between a warning limit and an action limit).
For Machine A, all the plotted sample means are well within the warning limits, let alone the action limits. The points appear to be randomly distributed around the target mass of 25g. There are no concerning trends or patterns. This indicates that the process is stable and "in control". Therefore, Luka does not need to take any action and should just continue monitoring.
Common mistakes
✗ Stating that action is needed just because the points are not exactly on the target line. Variation is expected.
✗ Misinterpreting the warning lines as action lines.
Practice the full AQA GCSE Statistics Higher Tier Paper 1
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