Quality assurance - CCEA GCSE Statistics guide to control charts, warning and action limits

Quality assurance is the Higher-tier application that uses statistics to keep a production process consistent. This CCEA GCSE Statistics guide covers sampling in quality control and how to read a control chart.

Sampling in quality control

In manufacturing you cannot test every item, so small samples are taken at regular intervals during production. A summary statistic, such as the sample mean to check the average and the sample range to check the consistency, is calculated and plotted on a control chart. This monitors the process over time and spots problems early, while testing samples rather than every item keeps quality control fast and affordable.

Control charts for the mean and range

A control chart plots the sample statistic against the sample number, with a central target line and two pairs of limits on each side. The mean chart monitors whether the process average is on target, and the range chart monitors the spread within each sample. The two work together, because a process can be on target on average yet becoming inconsistent, which only the range chart reveals. A fault in either the centre or the spread is then caught quickly.

Warning and action limits

The limits decide what action to take. Warning limits are drawn closer to the target, often where about 95 percent of samples fall when the process is in control, and a point beyond a warning limit is unusual and should be checked. Action limits are further out, often about 99.8 percent, and a point beyond an action limit means the process is out of control. So a point inside the warning limits means in control, a point in the warning zone means take another sample, and a point beyond an action limit means stop and adjust.

Reading a control chart

To interpret a plotted point, find which zone it falls in. Inside the warning limits the process is in control and no action is needed. Between a warning and an action limit, in the warning zone, take another sample soon to check whether the process is drifting. Beyond an action limit the process is out of control and must be stopped and adjusted. A run of points moving steadily towards a limit, even if still inside, is itself a warning that the process is drifting and should be investigated before it fails.

How CCEA examines quality assurance

CCEA rewards understanding sampling in quality control, the purpose of the mean and range charts, the meaning of warning and action limits, and recommending the correct action for a plotted point. The topic applies sampling, the mean, the range and the normal distribution from earlier in the course. Use the dot point for specification-level detail and worked CCEA-style questions, then test yourself with the quiz.