Using SPC Control Charts to Reduce AQL Sample Size

[Trisampson]

Hi all,

Our company (a medical component manufacturer) utilizes "In-Process" inspections during the manufacturing process as well as an AQL lot acceptance ("Final") prior to shipping. We would like to reduce, or eliminate the AQL sample size based on the In-Process data. If we are using SPC, are there rules or references available to guide us in reducing the AQL sample size? One route is to use process capability statistics (such as a > 1.33 Cpk reduces the AQL index to 2.5).

Has anyone any recommendations on this?

Thanks in advance...

[John Nabors]

Trisampson-

Originally Posted by Trisampson

Hi all,

One route is to use process capability statistics (such as a > 1.33 Cpk reduces the AQL index to 2.5).

Has anyone any recommendations on this?

Thanks in advance...

This is just one knuckehead's , but I would be very skittish about this. It would be one thing if you were making Barbie clothes or birthday candles but in medical I would be very leery of loosening your AQL's.

What does your customer(s) think of this?

Regards -John

[John Nabors]

Oh, and welcome to the Cove!!!!!

[bobdoering]

Check your contracts - most customers specify a required final inspection AQL prior to shipping. However, proper SPC will help you pass those inspections, and minimize any sorts, which are a bigger drain on the till.

[Steve Prevette]

Originally Posted by Trisampson

Hi all,

Our company (a medical component manufacturer) utilizes "In-Process" inspections during the manufacturing process as well as an AQL lot acceptance ("Final") prior to shipping. We would like to reduce, or eliminate the AQL sample size based on the In-Process data. If we are using SPC, are there rules or references available to guide us in reducing the AQL sample size? One route is to use process capability statistics (such as a > 1.33 Cpk reduces the AQL index to 2.5).

Has anyone any recommendations on this?

Thanks in advance...

One thing to consider is holding the AQL the same, but checking to see if you can drop from Normal to Reduced sampling.

[qualitytrec]

Originally Posted by Steve Prevette

One thing to consider is holding the AQL the same, but checking to see if you can drop from Normal to Reduced sampling.

I agree. Use the switching Procedures found in ANSI/ASQ Z1.4-2003. If things go poorly then you increase inspections if they go well you loosen them.

8. Normal, Tightened and Reduced Inspection

8.1 Initiation of Inspection. Normal inspection will be used at the start of inspection unless otherwise directed by the responsible authority.

8.2 Continuation of Inspection. Normal, tightened or reduced inspection shall continue unchanged on successive lots or batches except where the switching procedures given below require change.

8.3 Switching Procedures.

8.3.1 Normal to Tightened. When normal inspection is in effect, tightened inspection shall be instituted when 2 out of 5 or fewer consecutive lots or batches have been non-acceptable on original inspection (i.e., ignoring resubmitted lots or batches for this procedure).

8.3.2 Tightened to Normal. When tightened inspection is in effect, normal inspection shall be instituted when 5 consecutive lots or batches have been considered acceptable on original inspection.

8.3.3 Normal to Reduced. When normal inspection is in effect, reduced inspection shall be instituted providing that all of the following conditions are satisfied.

a.The precedinng 10 lots or batches (or more, as indicated by the note to Table VIII) have been on normal inspection and all have been accepted on original inspection; and

b.The total number of nonconforming units (or nonconformities) in the samples from the proceding 10 lots or batches (or such other number as was used for condition "a" above) is equal to or less than the acceptable limit number given in Table VIII (see 8.5). If double or multiple sampling is in use, all samples inspected should be included, not "first" samples only; and

c.Production is at a steady rate; and

d.Reduced inspection is considered desirable by the responsible authority.

8.3.4 Reduced to Normal. When reduced inspection is in effect, normal inspection shall be instituted if any of the following occur on original inspection:

a.A lot or batch is rejected; or

b.A lot or batch is considered accptable under the procedures for reduced inspection given in 10.1.4; or

c.Production becomes irregular or delayed; or

d.Other conditions warrant that normal inspections shall be instituted.

Hope this is helpful,
Mark