Making sets work for you

Sets are used to address a family of similar items. You need to decide when to use a single versus multiple sets.  There are cases when it is convenient to have an element in such a family and cases when it is not.

Suppose we have three grades of oil and three processes to crack it.  Should we have one set with nine entries or make the item two-dimensional.  I would do the latter.  Suppose we have a budget using fertilizer, seed, labor by month and water by month.  Do we have a set with 26 items or two sets one with fertilizer, seed, labor and water and the other with month names plus the label annual.  I would do the latter.  I err on the side of being more extensive with set definitions.

Sets should contain items treated similarly in the problem (i.e., resources like fertilizer, seed, and energy), but when there are two items crossed (i.e., monthly availability of land, labor, and water involves month and resource) one should have two sets.