UCD stat quantities P/S question

[Mark Taylor]

Dear semantics,

I have a question about some of the stat.* UCDs.

I see in the UCDlist document:

   S | stat.max
   S | stat.mean
   S | stat.median
   S | stat.min

but

   P | stat.stdev
   P | stat.variance

Why are max/min/mean/median designated as Secondary but
stdev and variance as Primary?  I can imagine wanting to use,
e.g. both

   pos.parallax;stat.mean

and

   pos.parallax;stat.stdev

but the second is apparently not allowed.
I was expecting all of these to be marked S or maybe Q.

Thanks for enlightenment,

Mark

--
Mark Taylor   Astronomical Programmer   Physics, Bristol University, UK
m.b.taylor at bris.ac.uk +44-117-9288776  http://www.star.bris.ac.uk/~mbt/