Not a definite fact, but I think I've read before that the number of individuals marshalling in any given year is around 1,500.

If that is the case then around a third were non locals who did 5 or more sessions. I think we ought to also bear in mind that the weather last year meant that Mad Sunday was the only day of race week without racing (6 different days) which would enable some over only for race week to get in the five sessions which they might not have done in an 'ordinary' year, perhaps inflating the numbers.

Again if the 1,500 figure is correct that leaves around 1,000 (probably more in a fine weather year) who either marshalled less than 5 sessions or are local (or both). My mindset opinion of marshals is that the non-locals probably do all sessions that they are on the island for which, would skew the numbers in favour of locals (but that is purely hypothesis and definitely not fact).

Happy for anyone to put me right with some facts.

EDIT: The TTMA website suggests around 1,200 so my theory above would be around 700 locals and those doing less than 5 sessions not 1,000.
http://www.iomttma.com/index.php?option=...&Itemid=16