Unfortunately, the probabilities  and  cannot be measured directly. Instead, we assay the relative population of folded molecules using various structural probes, e.g., absorbance at 287 nm (which reports on the solvent exposure of tryptophan and tyrosine), far-ultraviolet circular dichroism (180-250 nm, which reports on the secondary structure of the protein backbone), dual polarisation interferometry (which reports the molecular size and fold density) and near-ultraviolet fluorescence (which reports on changes in the environment of tryptophan and tyrosine).
The probabilities of folded molecules cannot be directly measured, so we use structural probes to estimate their relative abundance. These include absorbance at 287 nm (which measures solvent exposure of tryptophan and tyrosine), far-ultraviolet circular dichroism (180-250 nm, which reports on protein backbone secondary structure), dual polarization interferometry (which reports molecular size and fold density), and near-ultraviolet fluorescence (which reports changes in the environment of tryptophan and tyrosine).