How to Determine In-Process Sampling Strategies

What should you measure or avoid?

Determining an effective in-process sampling strategy can be a tricky business. What should you measure? What should your sample size be? What are the pitfalls? Your approach can be the determining factor to whether you will ever attain true understanding of process performance or see any significant improvements in quality, uptime, or deliverability at cost.

Developing sampling plans for acceptance sampling is typically a well-documented process based on industry-accepted standards and practices designed to detect if a lot meets an acceptable quality level. Most quality managers use acceptable-quality-level tables to determine the number of parts to sample from a given lot size. However, developing in-process sampling strategies is more than referring to tables; it requires an understanding of the manufacturing process, patterns of variability, historical stability of the process, and a willingness to use data to drive improvements.

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Comments

Wise-dom

Wise words, Mr. Wise, thank you. An all too common failure in quality management systems is to green-light incoming raw materials & components after an incoming sample-based inspection. When asked "why THAT sample size?" and "why THOSE controls?" the auditees' usual answer is "we have always done so". But very seldom, unfortunately, there's positive interaction, or exchange, with the processing - or production - processes. But there is where the problems arise. I don't blame quality people except for one thing: they keep looking at quality as at after-event, therefore control, instead of looking at it as before-event, therefore prevention, process. And, let's be honest, the ISO TC 176 certainly doesn't do really much to enhance prevention. Thank you.

Thanks Steve, Your post came

Thanks Steve, Your post came in such time when I was thinking on the same line regarding sampling size and frequency.

In my opinion, if you have put the interpretation of control chart with some examples and exactly how to derive sample size and frequency it would have nicest article.

Moreover how it could change based on situations would make it rich one.

If in forthcoming post if you could, I will wait for the same.

Thanks,

Mukundraj.

how to determine sampling rate and sample size

Maybe my paper on sampling rate and sample size in SPC may result helpful to some of the persons who commented. I would be very happy to receive feedback.

Here you can download: http://www.hrpub.org/journals/article_info.php?aid=1587

Regards

Textbook sample sizes

"Generally, most textbooks use sample sizes of 1, 3, 5, and 10." Can you give me a hint in which textbook I could find a rationale for these sample sizes?

Thanks!

Sample size of subgroups

It depends on:

1) What is possible to sample: Can you sample as much as you want?

2) The analysis cost: Do you have enough resources to pay for the analysis?

3) The analysis time: How fast do you have the results? The reason you do in process sampling is because you want to react on time if the value is out of tolerance. If you the analysis of 30 samples is not within a reasonable time to take corrective action and steer your process to be within tolerance, then you need to reduce the amount of samples.

4) How accurate you want your estimation to be? Look at the 1-alpha confidence interval of your point estimation for the average and the standard deviation. Remember the average is most probably normally distributed and the standard deviation follows a chi-square distribution. If both ranges of uncertainty are narrow enough for your application, then you choose that specific sample size. If you look at graphs that show the sample size versus de 95% confidence range for example, you can notice that goind higher than a sample size of 20 doesn't add much to lower the range.

I hope this type of evaluation helps you better than any value written in a textbook. If somebody asks you why you have chosen that value for the sample size, you can't answer "recommended in a textbook". It is better to validate it and record it in a validation document with reasoning. If someone asks "why?" then you can refer to the validation document.

Same goes for the sampling setup where you must define the sampling interval:

1) fixed interval and fixed sample size

2) varying interval and fixed sample size

3) fixed interval and varying sample size

4) varying interval and varying sample size