It IS important to note individual variation. If someone has a fever that's easily cured by a specific drug, but they tell you that they have a rare, fatal allergy to that medication, you don't give the drug to them anyway on the grounds that it's "unlikely" it'll kill them.

Similarly, if a particular drug is known not to have the 'normal' effect in a patient, you don't keep giving it to them in hopes that their bodies will suddenly begin acting differently.

The key is to distinguish between genuine feedback of failure, and rationalization. THIS POINT IS NOT ADDRESSED ENOUGH HERE. There are simple and effective means of identifying the difference between rationality and rationalization, but they are not discussed, they are not applied, and frankly they don't even seem to be known here at LW.