Frequently asked questions and answers
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Questions about getting started » New question!
Questions about control limits »
Questions about using SPC »
Question: In my company we are not fully in agreement about where in our production we should start using SPC. What do you recommend?
Answer: The most usual thing is that people want to start where there are stoppages, other problems or maybe a high reject rate.
That is a good idea, but because introducing SPC is a long-term project and is largely a matter of adopting a new mindset, we recommend instead that you reverse that procedure. Start where there are fewer problems; that makes it easier to get going. Let the new way of doing things get established and thereby set a good example.
Question: Everybody here keeps hearing all the time that SPC is something customers insist on, and that’s a good enough reason. But as the production manager, I need more good reasons to give my people. Suggestions?
Answer: These are some of the advantages that SPC brings:
- All the operators read the process the same way. There is less room left over for individual interpretations.
- Process know-how increases. The effect of this is often underestimated.
- Capability increases, which reduces hidden costs such as stoppages downstream in the production chain.
- There is usually less stress. Operators often realise that they have previously been measuring and adjusting their processes quite unnecessarily.
- You get fewer rejects.
- There is less need for final inspection; high-capability deliveries do not need an extra check.
- Fewer complaints and more satisfied customers.
Question: Our use of SPC keeps petering out. Do you have any recommendations?
Answer: A common mistake is to make everything far too complicated and thus cumbersome. Here are a few points to bear in mind:
- Start on a small scale and keep it simple.
- Make the whole thing less formidable by focusing on improvements (efficiency and customer utility) rather than on the method itself.
- Let the users themselves discover that SPC, in the great majority of cases, means less work, not more.
- Be tolerant of mistakes, especially at the outset.
- In our view, many software programs are too complicated and awkward to use. Choose a program that is easy to use and that helps machine operators to read the process clearly.
Question: A lot of people in our company see the introduction of SPC as something very complicated. What is the best way to get started?
Answer: Many manuals emphasise the need to prepare, to do an FMEA, to identify the properties where SPC can do the most good, etc. before you start to use it.
We do not agree; our view is based on practical experience. Starting to use SPC is a matter of adopting a new mindset, so you can quite well go ahead without doing all those other things.
Just be sure to start on a small scale. Keep it simple, support and encourage the users. Tolerate some mistakes in the control graphs – it isn’t the end of the world. Focus more on the application than on the method; the main thing is to get started. After a while the operators themselves will come to appreciate the value of SPC, and more will want to start using it.
Michael Nielsen’s advice on introducing SPC (PDF) »
Question: In our company there is a widespread view that we will “control ourselves to death” if we let control limits follow the process. What is your take on this?
Answer: This is probably the commonest misconception about statistical process control, and the usual reason is that people do not understand what the control limits are based on.
Because control limits are used with mean values and not individual readings, and because they follow the process automatically, one of the most usual effects of using SPC is that you realise that you have previously been measuring and at worst adjusting the process quite unnecessarily. Consequently, you will often wind up adjusting the process less often with SPC because you have realised that it does not need adjusting more often.
On the other hand: if the process is influenced by fixed, non-adjustable tools, it may be a good idea to lock the control limits. But in that case you must be aware that you are losing the link with the process.
Question: On one of our production lines the process capability is Cpk 0.9. We therefore have 100% inspection on that line. In my capacity as quality manager I argue that SPC can help us even when we inspect everything that is produced. What do you think?
Answer: We think you are quite right, for two reasons:
- The process will be centered as close as possible to the target value, which will result in the fewest possible rejects, i.e. the smallest possible reject costs.
- The control graph will immediately let you see the effect of any improvement measures you take, giving you fast and objective confirmation of whether they have done any good or not.
Question: I am the quality manager of a company whose customers do not demand that SPC be used. I want to introduce it nevertheless, but all the time they tell me I must give priority to production. What is your view of this?
Answer: There is no contradiction between SPC and productivity. In most cases SPC takes less time than traditional ways of doing things, and you get fewer rejects that have to be replaced by new parts, which effectively means that production increases.
We have even seen cases where poor process capability (caused by not using SPC) has adversely affected customer relations to such an extent that the customer stopped ordering.
Question: Can you give real-life examples of the benefits of using SPC?
Answer: Here are two concrete example of improvements to which SPC made a decisive contribution:
- In one machine group the result was that
- the annual reject rate fell from 7% to 2%;
- productivity increased somewhat;
- all the operators learned more about the process. Some of them learned a lot more.
- the customer stopped complaining about quality.
- A machine that had previously been regarded as utterly worthless was fully approved after machine capability studies, subsequent remedial measures and application of SPC:
- Process capability rose from Cpk 1.1 to Cpk 2.35;
- The cost was a three-day stoppage and ten hours pay for a toolmaker;
- The saving amounted to about one million dollars because there was no need to buy a new machine.
In addition to the above, SPC is used for example in such widely varying fields as plastic injection moulding, the woodworking industry, food production and health care.
