Control Charts – Seven Basic Quality Tools

Control Charts help visualize how processes are doing over time.

For example, assume you are the Quality Manager in a glass making factory. Now believe it or not, glass is made from liquid sand. You can make glass by heating ordinary sand (which is mostly made of silicon dioxide) until it melts and turns into a liquid. You won’t find that happening on your local beach because sand melts at the incredibly high temperature of 1700°C (3090°F).

As the Quality Manager you need to make sure that the temperature of 1700°C is maintained in the furnace. For quality purposes, you take several temperature readings over a 24 hour period and plot it on a graph. Below is what you get,

In the graph, the Lower Control Limit is 1600°C and the Upper Control Limit is 1800°C. The mean is somewhere around 1700°C which is the ideal temperature for glass making. When a data point falls outside the control limits we say that the data point is Out of Control. This also makes the entire process go out of control.

Now it is quite normal for data points to fluctuate but when seven consecutive data points fall on one side of the mean this means we have a problem at hand. This is also known as Rule of Seven. This concept is important from PMP exam point of view.

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