Exercise 63 Page 299

The quality of a product over time can vary in every manufacturing process, which is why statistical process control was implemented. A process control chart is used to plot the mean of the manufacturing process. This type of chart also includes three horizontal lines.

We can see these lines in this sample process control chart.

A manufacturing process is said to be in control if all means are within the process control limits. By this definition, the above process is in control since all data points fall within the lines. Now, let's move on to the process control charts at Katie’s Konfections. Here is the first chart.

Notice that four data points are above the UCL and four points are below the LCL.

This tells us that the process is out of control. A possible explanation for this is that inexperienced workers are operating on the machine, or perhaps there was a malfunction of the machine itself. Let's take a look at the second chart.

We see that this process is fully in control — the mean chocolate weights are all within the acceptable limits. Let's move on to the last chart.

This time we see that one of the means was below the LCL.

Technically, this process is out of control. We could suspect that one of the reasons we mentioned earlier might apply here as well — perhaps there was a machine malfunction or an inexperienced worker on the machine. Note that these reasons are only exmaples. We could think of many other reasons for a process to be out of control.