Two things we know.
(1) Scientific terminology is burdened with the baggage of history, which now makes no logical sense. So… early type galaxies are the ones with late type stars? Errr… And which of these terms relates to a sequence in time? Neither. Right. Very helpful.
(2) When you have to teach something, you finally figure out things that have been bugging you for years.
(3) Nobody expects the … oh. Anyway. Often (1) and (2) combine to make a particularly thick fog.
For some time the term “marginalisation” had been nagging at me, but I ignored it because I had other stuff to get on with. I am referring to the term in statistics. You have a probability density function of two variables, f(x,y), but decide that y is “interesting” and x is “uninteresting”. You then integrate over x to get a PDF p(y) for y alone. This known as “marginalising over x”.
So here is the quiz. My guess is only about seven people will want to take it, but I can’t resist it.
Rule (a) Andrew Liddle is not allowed because I already told him the answer in the pub. Rule (b) No Googling. Rule (c) Never talk about Stats Club.