I've been learning all these laws about Sekefot-doubts, but I'm coming up shy on a basic point. What is considered a Safek? Example:if there are 100 pieces of lettuce and I check all of them and only find one bug on one piece of lettuce, this means that 1/100 are the chances that I'll find another in my next batch of 99%. Is 1/100 a Safek Deroayta or is it just such a small chance that we say the Rov is too overpowering to Oser?
Contemporary poskim discuss how to approach statistics -- we tend to work with concepts of mi'ut hamatzui -- a "commonly occurring though less than 50% event", for which we do need to check; vs. mi'ut sh'eino matzui -- a rare event, for which we don't.
Many poskim treat 10% as the cutoff line for mi'ut sh'eino matzui, based on a fascinating application from the laws of Trumah, about when we need to check if wine has spoiled, to laws of sales, regarding the normal spoilage rates of wine in Talmudic times.
Furthermore we have the distinction of ruba d'ita kaman, i.e. we know there is a bug someplace in this bag, vs. ruba d'leita kaman, we know in a theoretical sense that occasionally there are bugs.
Rabbi JD Bleich addresses this in a Tradition article c. 2007: let's assume that 5% of dairy cows are treifot. That means that 5% of cows produce non-kosher milk. Now if I buy a bottle of milk made at a commercial dairy, it was mixed together from many different cows, meaning that on average 5% of this bottle is non-kosher -- and 5% is not batel! Answer: we apply ruba deleita kaman -- the statistic of 5% is an abstract one. Therefore, take any given cow, and we rule: "we assume this cow is kosher. 100%." Take the milk from that cow, and it's kosher. 100%.
Specifically with regards to miut, safek, hazaka and the like as applied to bugs in food, see Rabbi Eitam Henkin's book Lakhem Yihyeh Le-Okhlah.