The uncertainty in the free-energy difference ∆v(y) depends on the type and number of observables. For different types of observables probing different aspects of molecular conformations, we will have different expectations about the energy error. For a polymer, the expected energy uncertainties for sub-ensembles with fixed end-toend distance will be different than for sub-ensembles with fixed carbon-hydrogen bond lengths. The sub-ensembles for these two observables are quite different and so is our energy uncertainty.