From a case of lettuce (which is generally 24 heads), take three heads. Discard the outer leaves of each head. (Reason: The outer leaves are the most infested, and removing the outer leaves reduces the probability of infestation in the head of lettuce.)
Check all the remaining leaves of each head.
If even one bug is found in the three heads, all leaves of all remaining heads must be checked.
If, on the other hand, no infestation is found in the first three heads, all of the remaining heads of lettuce may be eaten without checking25.
This method is difficult to justify, and is in fact rejected by most kashruth organizations. Once a substance is in the category of sometimes infested (Hebrew: mee-oot hammasui), it may not be eaten until it is properly checked. Checking three heads and finding them to be clean―does not alter the halachic status of the item.
Perhaps a variation of this method is more sound. As follows…
Method Two 26
1. Soak all of the individual leaves of all the heads of lettuce, in vegetable wash or some other soapy solution.
2. Agitate each leaf in the water.
3. Remove the leaves from the water, and check the water for infestation.
If there are signs of infestation in the water―empty the water and repeat the process. Do this as many times as necessary―until no infestation is seen in the water.
Then, proceed to the hazakah check. Check three heads of the lettuce. If no infestation is found, the rest of the lettuce can be used without checking.
If infestation is found—all the lettuce must be checked26.
Method two has an advantage over method one: We have changed the status of the lettuce by soaking it. If we had found infestation―we would have continued to soak the lettuce until the infestation was no longer there. Consequently, checking three heads in addition to this makes us reasonably certain that there is no infestation in the lettuce.