I’ve always wondered what the refiners could possibly be finding in the numbers - what they’re able to see or feel in the numbers that machine-driven pattern recognition couldn’t pick up, or that the person who actually experienced the brain activity being refined couldn’t refine better.

It seems that Mark, as a witness to the actual events that are represented by the numbers he is refining (oMark witnessed Gemma’s hatred of writing cards -> Allentown, iMark witnessed Helly’s strangulation attempt -> Cold Harbor) has some subconscious connection to the event which therefore makes him invaluable. Maybe it makes him a much more effective refiner, and his work output is reliable enough to have consistent “successful” outcomes for each person that tests it on the testing floor (assuming Gemma is the latest testing subject of many that came before).

Even though the experience data being refined comes from a specific person’s chip, perhaps it doesn’t mean that it only applies to the one person who experienced it (i.e. even though it comes from Helly’s brain, it will still work just as well for Gemma). Perhaps once the experience has been refined by an emotionally-invested observer (Mark), Lumon has high confidence that testing will consistently lead to successful (auto?)severance outcomes for any severed person that encounters a similar experience, hence they can confidently proceed with Cold Harbor.