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.

- For each sample, 2 positive control cells measurements and 2 negative control cells measurements.

Option 1) Positive measurements are averaged and negative measurements are averaged. I finally obtain 1 result (average positive : average negative) per sample.

So I have n results. And the standard deviation from these n results.

Option 2) All 4 ratios are calculated (positve result 1 : negative result 1, positive result 2 : negative result 1, positive result 1 : negative result 2, positive result 2 : negative result 2).

So I have 4*n results. And the standard deviation from these 4*n results.

Then I'd like to use the following formula for pooled standard deviation to determine the known variability:

s = sqrt (((n1-1)s1^2+(n2-1)s2^2+...(nk-1)sk^2))/(n1+n2+....nk-k))

with k the total number of batches.

Do you think I can apply this formula?

If so, which option (1 or 2) above do you think is the best to get s1, s2, ... sk?

Thank you