Set membership H-infinity identification is investigated using time-domain data and mixed parametric and nonparametric models as well as supposing power bounded measurement errors, The problem of optimally estimating the unknown parameters and evaluating the minimal worst case identification error, called radius of information, is solved, For classes of models affine in the parameters, the radius of information is obtained as function of the H-infinity norm of the unmodeled dynamics. A method is given for estimating this norm from the available data and some general a priori information on the unmodeled dynamics, thus allowing the actual evaluation of the radius of information. The radius represents a measure of the "predictive ability" of the considered class of models, and it is then used for comparing the quality of different classes of models and for the order selection of their parametric part. The effectiveness of the proposed procedure is tested on some numerical examples and compared with standard statistical criteria for model order selection.