I am not sure if there was such topic but I couldn't find it and really hope that someone will be able to help me out!
I am working with dried flaked herbs and I am in the middle of building our testing regime. Every batch (approx 200kg) is tested for microbes and allergens (our customer's requirement) but only one sample (100g) is sent out. Do you think I need to send more samples in order to assure product is free from allergens and contaminants or one sample (picked from different barrels of one batch) is good enough? I understand that 100g doesn't represent 200kg, so I was wondering if anyone works with similar products and could help me out?
Yr question is a classic sampling query. It is a similar one to people who wish to be confident that their collection (lot) of packed cartons does not contain Salmonella.
The short answer to yr question is that the number of samples taken from a lot, not so much the size of the lot, dictates the degree of confidence with which one can assert that the percentage of units in the batch exhibiting a target characteristic X is less than Y%. Generally the size of the lot is not so important if much larger than the sample size (eg > approx. 10x from memory).
For example if yr lot contained 10,000 boxes of 25g each and you took 10 random boxes to test for X, a negative result for all samples would mean that you can assert with a 95% confidence that in the whole lot, the percentage Y of boxes containing X is less than 30% !. (100 negative samples would give a Y of 3%.)
The above assumes a uniform distribution of X throughout the lot which is unlikely in practice so that the above comments may be somewhat optimistic.
To put it another way, if you believe that the lot contains 4% defective units, how many samples do you have to take to have a 95% probability of finding one defective unit ? The answer is 75 !
As you can see sampling to detect low levels of a defect with a decent probability of success requires many, many samples. Conversely a satisfactory (negative) result for a small number of samples does not really prove very much.
Basically yr OP needs a decision as to how precise/confident you wish to be that the lot is "free" of allergens.