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.