What is the word for the values derived from an ordered set such that the values divide (by virtue of their positions; not by their value) the set into 3 subsets that have an equal or nearly equal number of members? What is the word when the values are drawn from the set?
What is the generic word that applies regardless of the number of subsets?
EDIT: The parenthetical "by virtue of their positions" is probably unnecessary because, as I said, the set is ordered and the subsets have a (nearly) equal number of members.
EDIT: Four answers are sought: a word for a value in the case of 3 subsets, a word for a datapoint in the case of 3 subsets, a word for a value in the general case, a word for a datapoint in the general case.