A control chart is one of the 7 fundamental quality control tools. It can be referred to as the Shewhart chart or the process-behaviour chart, but they are all the same thing; a control chart.

The control chart is a means of showing whether or not any process is stable. When it is stable, then it will be under control. However, when any process is done over and over again, then there may be times when it does not repeat exactly and the control chart is a type of visual management that can display the results.

Processes may have slight variations and the control chart can show whether or not the variances derive from internally ie from sources that are in essence common to the process. If the chart shows that the process is not stable, then the chart can help to pinpoint the causes and sources of the variances, so that they can be eliminated.

The control chart will have 3 horizontal lines, namely the Upper Control Limit (UCL) the Average Line (often called the Mean) and the Lower Control Limit (LCL).

The Average line, the mean is the central line. The other two lines are at 3 average deviations at each side of the Average Line. Obviously, the more stable a process is, the closer to the Average Line it will be.

Calculations Of Control

Within the chart there has to be a method of calculating the control limits. Variables such as time, weight or even voltage can be used. If the question ‘How much?’ is asked, then a variable will be used as the measurement. Any non-variable measurements are referred to as attributes and these are the numbers of an item, such as the number of defective goods.

However, when a point is shown on a control chart, it is usually comprised of the average or mean of a set of measurements. This is due to statistical reasons, for predicting the distribution but also enables much tighter limits to be achieved in terms of control. Averaging a number out ensures that any individual high or low measurements can be smoothed out. This results in the control chart being able to reflect any very small changes in any given process. If the points were all individual numbers, there would be too many to present a realistic picture and it would be incredibly hard to accurately pinpoint any tiny changes.

Tracking Variables

When tracking variables, it is necessary to use two control charts, simply because if only the average of some subgroups were measured, there could be a really significant variance within the different subgroups that could be easily missed. So in order to accurately track the variances, two control charts need to be undertaken.

Use of the Control Chart

The control chart is an exceptionally useful tool, particularly within the manufacturing business, simply because it enables manufacturers to be able to analyse objectively, the behaviour of any process. Although we may think that creating a ‘widget’ 1000 times an hour, in exactly the same way will lead to the exact process being repeated, there will be slight variations. These may be slight, but it is important to chart them so that they can be kept within ‘control’ because if the process deviates from control limits, the machines could be affected, the defect rate could be high and so on.

In essence, the control chart helps keep any process variations exceptionally low and they pick up on any changes very quickly. This ensures that any potential problems can be solved before they become problems, which is the goal of any quality tool and one which the control chart does exceptionally well!

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