Is there a simple calculation which can evaluate whether a number is odd or even?

Yes, it is called Modulo, or MOD for short. It returns the remainder of the integer division of a number. Lets say you have a number

field called 'inum'. MOD(inum,2) will return either a 1 or a zero. If you would like the expression to return either 1 or 2, do the following: MOD(inum-1,2)+1. Since this is integer division and really only the remainder, it doesn't make any difference how big inum is:

1001 returns 1, 1002 return 2 (if you use the -1 and +1 expression).

Thank you for the fast reply. Just what I needed.

This calculation can also be used to differentiate odd and even weeks if the number involved is a week(date) calculation.

e.g - assuming field week number is week(date) calc

Case(mod(week number;2) = 1; "odd week";"even week")

This is useful if you have to set startup scripts do things on a fortnightly basis.

(Just thought I'd throw it in for anyone who is interested)