We suggest to use our prior knowledge about the forcefield accuracy in the space of observables to set the confidence parameter as θ ∼ = 2/var(β∆U ), Eq. (1). One can justifiably deviate from this proposal to set θ according to available prior knowledge. For example, if larger values of the variance are less likely than implied by the exponential form of the prior with θ ∼ = 2/var(β∆U ), then one should increase θ accordingly. In any case, the knowledge of the expected variance var(β∆U ) can be applied to set and interpret the scaling parameter θ. Conversely, a particular choice of θ by other reasoning is also an expression of the expected errors in state populations of the reference ensemble. Whereas functions other than an exponential function could be used to define the prior, the simple relation to the force-field error would most likely be lost.