Comparison of the LENSTRONOMY lens model parameters and Fermat potential difference between the power-law (red) and composite (blue) mass models. The posteriors are obtained by averaging over all the model settings following the procedure described in Sect. 6.3. The parameter γ for the composite model is computed from circularly averaging the quantity 2 − [d log α(r)/d log r]_r = θE. Some parameters are blinded as for p ∈ {γ,  Δϕ}, where the subscript ‘pl’ refers to the power-law model posteriors. The composite model-predicted γ is approximately 8% smaller than that from the power law. Consequently, the Fermat potential differences are smaller by approximately 16% for the composite model.