the problem of assigning classes to observations is very common in
other areas too, e.g. computer vision.
I agree with Barry that binary decision trees are problematic since
one property/attribute, which might vary widely or is difficult to
get hold off may influence the decision strongly.
However, any binary (observation yes/no) classification schema like
the proposed decision tables (Barry) suffer that attributes either
vary widely or that observation are difficult to obtain, imprecise
or subjective. It also does take not into account that different species
are sometimes so closely related that it is hard to make distinction
among them.
Computer vision, expert systems etc. have recognised that problem and
use different techniques to handle imprecise or varying inputs, e.g.
fuzzy classification, possibility/probability theory etc.
I wondering if eg fuzzy classification has been tried in botanical
taxonomy.
--Michael