Forum for Sampling Practices

Hi,
I am responsible for designing quality control sampling plans for diagnostic kits.
I have been using a sampling plan by variable for unknown variability so far but would like to switch to known variability in order to decrease sample size.

Several batches of products have been inspected so I can use all these data.

However I am not sure what is the best way to calculate the standard deviation.

Here is what I have:
- For each batch, lot size N, sample size n.



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saramaadeva wrote:Hi,
I am responsible for designing quality control sampling plans for diagnostic kits.
I have been using a sampling plan by variable for unknown variability so far but would like to switch to known variability in order to decrease sample size.

Several batches of products have been inspected so I can use all these data.

However I am not sure what is the best way to calculate the standard deviation.

Here is what I have:
- For each batch, lot size N, sample size n.

How many batches, typical lot size, typical sample size? You will require a much smaller sample size with the known standard deviation plan.

The correct standard deviation to use is the pooled within-lot standard deviation. You can find many examples and discussions of the formula if you Google "pooled standard deviation."

If a sequential sampling plan is practical for your operation, a TSS sequential sampling plan for variables will require a smaller average sample size yet (matching the same OC curve.) Software program TP414 is the only program that I know of that can calculate a TSS (It will also match a fixed-n to any OC Curve.) http://samplingplans.com/software_oc.htm

Stan Hilliard

saramaaedeva wrote:Hi,
I am responsible for recommending these affordable weight loss pills for men and designing quality control sampling plans for diagnostic kits.
I have been using a sampling plan by variable for unknown variability so far but would like to switch to known variability in order to decrease sample size.

Hi Stan, are there any guidelines on how to roughly estimate this by any chance? Cheers.