Using Fixed Sampling procedure with SPC

[Former Member]

Dears

According to SAP help, it's stated that the sampling procedure which should be used with SPC must be fixed sample http://help.sap.com/saphelp_40b/helpdata/en/18/4389fd449011d189420000e829fbbd/frameset.htm

Please provide me with the reasons for using only fixed sample why not to use percentage sample for example

[former_member42743 · Accepted solution]

SPC is typically used for in process control, for 03 inspection lots.  Folks often want to use SPC for finished product testing but it's technically not really applicable there.  SPC does stand for Statistical Process Control.

During in process testing, your quantities might not even be known, especially in the chemical industry where yield can vary and is often not even know until final pack-out.

During in process testing, if you are doing real SPC, and you take a sample every 30 minutes, the sample size of each of those 30 min samples should be equivalent.  True SPC is used for controlling your process, not really to demonstrate your final product quality.

If you are manufacturing widgets, again, you don't want to take the average of 5 at one time point, and then take the average of 6 the next time period, and then 4 the next.  Statistically I don't think most procedures would support that.  But I'm not a statistician so I could be wrong with that.

If you try to use an SPC characteristic on final results, folks are usually looking at these for run history reports for customers.  So you want a chart of the %solids for all tank trucks sent to customer XYZ.  You really can only have one value for each point in these cases.  You want a value representative of the truck.  If you take three samples one time, and two sample another time, again, for a pure analytical viewpoint, you aren't comparing apples to apples.  It's almost like using two different test methods to the same test.

FF

[Former Member · Accepted solution]

Dear

First of all I would like to thank you for your answer but i have some concerns regarding your comment,

As you stated above folks often wants to use SPC with finished product testing, this is the case in my project. What we need to do is using percentage sample (Ex. 5%), it's not reasonable to take fixed sample whatever the quantity received.

Business wise it's applicable ("np" charts are used with fixed sample and "p" charts are used with variable samples) so why not to use this in SAP???!!!

[former_member42743 · Accepted solution]

Well.. I can't say exactly why SAP does it the way they do.  I can only give you my thoughts.

But what I would argue is that a true SPC is not used for finished testing.  Run charts are available in SAP for this.  True SPC is meant to give feed back to an continuous process.  It's why it's called a process control characteristic.  They were meant to allow you continue production as long at the process was under control.  Action limits are set up to allow tweaking of the production process to keep it in control.  Exceeding the limits can result in a number of outcomes depending on the process.  Creating a new distinct batch for one, shutting down and cleaning out for another.

But what many people and firms have done is apply the same charts to finished product testing.  I've had clients that made every characteristic an SPC characteristic so they could "report" on anything.

When you apply SPC calculations to finished product testing, all you are really doing is a statistical analysis on a data set.  So I don't think SAP ever really tweaked this for 04 lots.

Even p charts are meant to be done on in-process testing, (i..e the 03 lots).

But.. have you tried to change the settings in config?  In standard SAP all the control charts are delivered requiring a "fixed scope".  This is in table QPSH.   This can be changed.

You can get to the config via

Quality Management-->Quality Planning-->Sample, SPC-->Statistical Process Control-->Define Control Chart Type

Double-click on the chart you want to change.

In the next screen you can change the scope indicator from Fixed to scope not fixed or try clicking on multiple samples.

I'd experiment with a few of the settings and see if any of these will work for you.  Just remember that SAP will probably not be able to give you much help if something doesn't work as you expect.  You'll have to debug it yourself and verify that the calculations you get are appropriate for the data your using.

I'd be interested to hear how it works out for you and if by changing these settings you were able to achieve what you wanted.  I've just never had a client that wanted anything much different that what was already provided so I've never experimented with the settings there.

FF

[Former Member · Accepted solution]

Dear FF

Thank you for your patience and for your interest unfortunately it's not related to the control chart type, the error shown before choosing the control chart.

I am trying to find out a solution for this case, I will update you if I can have any solution

Many thanks

[Former Member · Accepted solution]

Dear FF,

Thank you again for your help, unfortunately till now i can't find the reason SAP restrict SPC to fixed sample only, but I have discussed a work around with my customer and it works , It was agreed to do it with fixed sample and maintain inspection points for the 04 inspection so that for example for each pallet we have fixed sample so that it's something like percentage

Thanks a lot