How are control charts used in quality assurance to monitor a process, and what do the warning and action limits mean?
Understand quality assurance and use control charts for the sample mean and range, interpreting warning and action limits to decide when a process is in control or needs adjusting.
A CCEA GCSE Statistics answer on quality assurance: sampling in quality control, control charts for the sample mean and range, warning and action limits, and deciding when a process is in or out of control.
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What this dot point is asking
Quality assurance uses statistics to keep a production process consistent. At Higher tier CCEA expects you to understand how samples are taken during production, how a control chart for the sample mean and the sample range is used, and how to interpret warning and action limits to decide whether a process is in control or needs adjusting. The skill is reading a plotted point against the limits and stating the correct action.
Sampling in quality control
In manufacturing you cannot test every item, so small samples are taken at regular intervals during production and a summary statistic is plotted to monitor the process over time.
Taking samples rather than testing everything keeps quality control fast and affordable, while still detecting problems early.
Control charts for the mean and range
A control chart plots the sample statistic against the sample number (time), with horizontal lines for the target value and the limits.
The two charts work together: the mean chart catches a drift in the average, and the range chart catches an increase in variability. A process can be on target on average yet becoming inconsistent, which only the range chart reveals.
Interpreting the limits
The position of a plotted point tells you what to do.
A trend of points moving steadily towards a limit, even if still inside, is itself a warning that the process is drifting and should be investigated.
Why this matters
Quality assurance is how factories, food producers and laboratories keep their output reliable, and control charts are used across industry exactly as the specification describes. The topic ties together sampling, the mean, the range and the normal distribution (the limits are based on how far samples spread when a process is in control), so it is a fitting application of the whole course. Reading a control chart and recommending the correct action is a clear, reliable Higher-tier mark.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA-style4 marksA control chart for the sample mean has a target of 500 g, warning limits at 495 g and 505 g, and action limits at 490 g and 510 g. A sample mean of 507 g is plotted. State what this indicates and what action should be taken.Show worked answer →
The value 507 g lies between the upper warning limit (505 g) and the upper action limit (510 g).
This indicates the process may be drifting: the sample is in the warning zone, so the result is unusual but not yet a failure. Two marks (locating it between the limits, identifying the warning zone).
Action: take another sample soon to check. If the next sample mean is also beyond a warning limit, or any sample reaches an action limit, the process should be stopped and adjusted. Two marks (continue monitoring, adjust if it worsens). A value inside the warning limits would mean the process is in control.
CCEA-style3 marksExplain why a control chart for the range is used alongside a control chart for the mean in quality control.Show worked answer →
The mean chart monitors whether the process is on target (the average value is correct), but it does not detect changes in variability. One mark.
The range chart monitors the spread within each sample: if the range increases, the process has become more variable or inconsistent, even if the mean is still on target. One mark.
Using both together checks that the process is both correctly centred and consistent, so a fault in either the average or the spread is caught. One mark.
Related dot points
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Sources & how we know this
- CCEA GCSE Statistics (2017) specification (2260) — CCEA (2017)