The value of sigma~a~ used in constructing the Luzzati plot for
reflections classified as observed. Details of the
estimation of sigma~a~ 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.