The value of sigmaa used in constructing the Luzzati plot for
               the reflections treated as a test set during refinement.
               Details of the estimation of sigmaa can be specified
               in _refine_analyze.Luzzati_sigma_a_free_details.
               Ref:  Luzzati, V. (1952). Traitement statistique des erreurs
               dans la determination des structures cristallines. Acta
               Cryst. 5, 802-810.