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