Nothing in writing. That ought to be fun when doctrines get corrupted down the line. They tell the same message to those near the top, who mess it up a bit. As the message moves down the line, these errors add up, so the message the publishers receive in one congregation could be different from that received elsewhere. And all could differ from the original, and they would have no way to verify anything.
And that is with just honest mistakes. Throw in a few deliberate corruptions, usually by hounders or hounder-hounders that are tyrants trying to grab as much power or money (usually both) from people as possible, and this message could be way worse than it ever was intended to be.
And no written door to door records means no return visits. It is easy enough to lose calls with written records. What happens when you find no one at home? It is easy to lose those slips. Now, they have to guess. Someone comes out in field circus that wasn't out during the initial calls in that territory, and they have to guess who was at home and who was not. This is going to result in excessive numbers of mistakes, calling on people that were not interested the first time and not hitting those who were simply not home or were busy the first time. I hope that, between the confusion of messages coming from the top and messing up with house to house records, they end up having precisely zero new studies.