How can I manage the defective rate in this situation?

I have a big question regarding the management of the defective rate(SPC management for attribute data). For variable data, I know we can use X-bar R Chart to do the SPC management. However when it comes to attribute data, it has become tricky. In my original plan I was going to use P chart to do the SPC management. The question is when using p char algorithm (UCL = p-bar + 3* sqtr((1-p-bar)*p-bar/n)) to calculate the UCL, the UCL is so small, almost 10 out of 35 of the points(defective rates) will be above the UCL. It indicates not only the whole process is not statistically stable, but also it is hopeless to do the P chart management. How can I do it with so many anomalies?! Before I was trying to use P chart to manage the defective rate, normally the algorithm for us to caculate the UCL for defective rate is that UCL = average defective rate + 3 * standard deviation of all defective rates. This UCL is large enough that there are not too many anomalies for us to check. The management on this UCL is doable. I know the algorithm of this UCL lacks the correct statistics support, however it can be implemented.
Please help me on this problem, thank you.

 

g're'a

Sir, it is the greatest answer I ever received regarding SPC management so far. Thank you very much.
"A control chart actually is designed to detect non-homogeneity." You kind of show me the essence of the SPC. Yes, it is all about "homogeneity". Sometime the process just has no homogeneity. The conventional methods simply can't be used in these cases. Or maybe the process being studied is just unstable.
In my case the sample size is always around several thousands pcs and the occurrence rate is definitely below 5%, average occurrence rate is from several thousands to several hundreds of thousands of PPM. We do the full inspection on every production lot by auto inspection machines.
With regards to the method I talked about, yes, I agree it would hide some anomalies and can't detect all the non-homogeneity. However even using this method, there are enough anomalies detected for us to analyse each day. We are not sampling checking some of the production lots. Instead we check each of them... There will definitely be a lot of anomaly lots detected each day, since we process many of them everyday. I want to know what would you do if you are in my situation. Thanks.

 

Once you tried Bev D's recommendation, can you post the results of both p" chart and ImR chart here? So we can see the difference.

 

I have tried the MR chart. The result is rather undesirable - there are too many OOC points. Regarding the P' chart he mentioned, it needs dollar to see the real content. Although I really want to read it, the payment process is quite troublesome for me and the price is a little beyond my allowance.

I have been working on SPC management for a while (focus on the deffect rate). Well I have to admit that it is a not standard SPC which strickly follows the book.(You can see the algorithm how I MANAGE it. UCL = average defect rate + 3 * standard deviation of all defect rates).

As a quality engineer, I tend to focus more on the data, on the trend. However, in the reality, I also need to take into consideration the processing capacity of the manafacturing line. Normally when the computer-aided quality system identifes an ABNORMAL lot, the system will automatically "HOLD" the lot from going downstream and stop the corresponding manufacturing machine if necessary. I NEED TO CONTROL THE NUMBER OF OOC LOTs. If there are too many of them, the production will be interrupted severely which also poses a BIG quality risk and there is also not enough manpower to deal with them.

So for me balancing the number of OOCs and the actual capacity is also a key of SPC management. Sometimes I just doubt that whether there is a need for a decent SPC management in my factory. ㅜ.ㅜ

 

Thank you for your help. I will do some serious study on the content you recommend.

I'd like to share some data, however my company have done a really good job on the protection for "corporate" intellectual property. And I also has a doubt that there is a huge fundamental misunderstanding towards the usage of SPC in my company.

I will take some time to think through it and hopefully to make a post to illustrate the detailed situation where I am.
PS: Sometimes I just hate the rigidity of the huge organization.

 

The following is the reason why I need to control the quantity of OOC lots.
(a really long message, please be patient to finish it.)

First I need to talk about the usage of SPC in my company.

As the Manual mentions, SPC control charts, both for attributes and variables, are the tools to determine whether the manufacturing process is stable or not by identifying whether there is out-of-control point(s) or abnormal trends. If there is such thing identified, the action shall be taken to stabilize the whole process and achieve the continuous improvement.

In my company, the output (i.e. products) from the process on which we are doing the SPC is subjected to full inspection, and the purpose is to identify the out-of-control point(s) which is OOC lot(s). The engineers will check these lots to:
1) see whether there is any underkill product (defective product which is not screened out by the full inspection) in this lot. If there is any, mark them out and scrap them to prevent nonconforming products shipment.
2) find the cause for the abnormality, do the correction to prevent more from happening. (Bring down Quality failure cost)

In my company every production lot will go through SPC control. Furthermore, we do the SPC control on all the main defect types (such as overflow, underflow, foreign material etc.) of each lot in each inspection station, not just on the nonconforming rate. Plus, though our production capacity is far below the average capacity of the competitors, still we produce more than 800 lots per day, and each lot wil go through at least two full inspection stations , and there are several main defect types in each station. It results in a lot of SPC charts and each chart has a lot of points.

As the manual said, the environment must be responsive. We actually establish this responsive environmnet by integrating the SPC into the MES. If OOC lot happens, the lot will be contained by the MES, and CAN NOT go downstream. And based on the setting, the corresponding manufacturing machine will be also stopped. The engineers need to go through the "RELEASE" process to resume production of the machine and let the lot FLOW again. IF THERE ARE TOO MANY OOC LOTs, the production may be literally stopped or severely interrupted.

That is why I need to control the quantity of OOC lots.