Control Charts
Hello colleagues and welcome back to Week-39 of the Weekly Lean Series. Today we are going to discuss a very familiar topic, namely Control Charts which we use to understand, monitor and intervene processes .
A process is a mechanism to create a desired output. If you are producing parts, for instance, your desired output has a definition , which is the quality standard. A standard can be about dimensions, color, functionality etc. which aims to ensure the customer is offered a quality product. Processes include variation and this can affect the desired characteristics of the product being manufactured. In a sense the control charts give us the capability to visualize, control, understand and intervene this variation to ensure quality .
Suppose we have got two machines producing the same part and we wanted to understand a critical characteristic of a part is correct in our manufacturing process output. So we collect samples from manufacturing and try to understand the variation in time.
We are using here an Xbar-R chart. This chart is suitable for measurement of a subgroup of samples. When we check the graph for the first machine we can see that there is a specific cause of variation at observation 8 for the part dimension. The variation looks in general under control from R chart. Therefore we need to check what happened at that time to understand the root cause.
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Looking at machine 2 we see that there are no points out of control. This means there are no non-random variation; meaning definite, specific causes that are called assignable causes. The R chart shows that the variation is stable as well as in Machine-1
There are many different control charts aiming to understand processes in different ways. You can check the table here to see which can be used when depending on data and sample collection strategies.
In a nutshell, control charts are reliable tools of Statistical Process Control that enable us to ensure a statistical understanding of our process and guide us to do the necessary changes to eliminate root causes of problems in our PDCA cycle.