Good point Ray. But the survey also reveals a weakness in these types of surveys. It could be, that people who are inclined to do drone brood removal for mite control, were less concerned about the hives that were obviously booming, but did it more on hives they felt were under mite pressure. And these hives could be more likely to end up dead, drone removal, or not.
Also, I suspect that the numbers involved may not have been big enough to give a low margin of error.
None the less, not criticising the survey in any way, the more people take part in it, the more useful it is.