  To learn about a physical system of interest, experimental results must be
able to discriminate among models. We introduce a geometrical measure to
quantify the distance between models for pseudoscalar-meson photoproduction in
amplitude space. Experimental observables, with finite accuracy, map to
probability distributions in amplitude space, and the characteristic width
scale of such distributions needs to be smaller than the distance between
models if the observable data are going to be useful. We therefore also
introduce a method for evaluating probability distributions in amplitude space
that arise as a result of one or more measurements, and show how one can use
this to determine what further measurements are going to be necessary to be
able to discriminate among models.
