Eva Lu Ator, another user, has also noticed the different intervals computed by different but algebraically equivalent expressions. She says that a formula to compute with intervals using Alyssa's system will produce tighter error bounds if it can be written in such a form that no variable that represents an uncertain number is repeated. Thus, she says, par2 is a "better" program for parallel resistances than par1. Is she right? Why?
Eva is right: Alyssa's system treats each repetition as a new, independent draw from the interval, inflating the final percentage tolerance of the calculation. Alyssa's logic has not fully abstracted syntax from semantics, as logically equivalent expressions do not yield the same value.