I thought this subject was researched by some other body a few years ago. They came up with a figure of around 25%, if I remember rightly. If I am correct, there are clearly some wide margins of error when this issue is researched. Research results can, of course, sometimes depend on why the research is being conducted. Research designed to show the existence of a major problem regularly produces higher percentages than rsearch which is not, even when the researchers are highly reputable. Sometimes, when their research has demonstrated nothing, they will latch on to some minor point in it and say "this is a potentially significant finding, which merits further study". In other words, "give us more money". When your job is substantially funded by handouts, it is a way of keeping you in business. And you might discover something that makes you world-famous one day!