NSEU I suppose it really depends on your definition of line and main base. Our aircraft return to a main base every day, but we work them on the line, so only occasionally do they go down to the base for a hangar input. We handle around 10 a/c per day. If a particular aircraft has significantly more oxygen servicing it becomes "common knowledge" among the engineers. Also if defect coding has been carried out correctly it's easy to search on our particular computer system for any oxygen related entries making it easy to identify a trend. So in a way we do have a good idea whats normal or not.