A courier hub fitted new sorting racks in January, and the number of parcels entered in its register as mis-sent fell sharply over the six months that followed. The hub supervisor treats this as evidence that the racks have reduced sorting mistakes. Which of the following, if true, most weakens the supervisor's reasoning?

The mistakes that made up most of the earlier count are simply no longer being counted here; they have moved to another register, not out of existence. The fall in this hub's figure is fully accounted for by that transfer, so it is no evidence at all about how well the racks work. The supplier's identity, a few days of unfamiliarity and December volumes leave the register's contents untouched.